hey
OK, firstly there is nothing wrong with DJing top 40 style music
Although all the “proper” DJ’s will look down on you and be all snobbish about their EDM or D&B/Dub or whatever, at the end of the day how many of them actually make a living from DJing and how many are just bedroom DJ’s? Totally nothing wrong with doing a little top 40 to pay the bills and still doing the EDM for your pleasure. DJing in student clubs has been my bread and butter for 12 years now and while I still do odd house gigs and other things for pleasure they’re what pay the bills 
Now in terms of mixing, the honest truth is with the type of music your playing no one will even notice. For 50% of the time a simple fade over and drop the next one as the previous fades out will be more than enough, and for the other 50% they will be similar enough in tempo that you may be able to loop the first few beats and do a wee medium speed beatmatch/fadeover but that is about as fancy as you’ll get.
Doing any more than this will actually go against you. These people are just here to hear songs that they know from the charts, get a bit drunk and dance silly, and any fancy mixing or cutting back and fourth or layering up multiple songs/samples will just put them off to be honest.
What you’ll have to concentrate on WAY above your mixing will be song choice. I could sit here all day writing lists of songs that would all go down well, but will give you some examples and you can take it from there.
Do it in wee mini sets, like 4 or 5 songs of a similar age/style/genre and then change to something else, maybe with one bridging song in between to smooth it over if its gonna be a big jump in genre/style, although in saying that big jumps can sometimes work well just situational really.
(Note this selection may be a bit UK biased in terms of what was popular in the charts for the US readers, but the OP is in Ireland so mostly should apply )
(PS, a good shout is to download the full “Now thats what I call music” catalogue, something like 70 disks they’re up to now and it has ALL of the chart hits back to the early 80’s. It’ll give you 75% of the tracks you need and you’ll just have to get some 60’s and 70’s stuff and your done.)
OK so what your gonna need is firstly a big selection of 60’s/70’s/80’s stuff this will be on average 40% of your night. Talking First last everything (Barry White), lots of ABBA, Bee Gees, Aretha, Love Shack, Love really hurts without you, some Blondie, Queen (mainly Dont Stop Me Now), boy george, young hearts run free, girls just wanna have fun, american pie (original) brown eyed girl, some of a preacher man, Jackson 5, Grease Megamix, Dirty dancing soundtrack, Elton John, Footloose, James Brown, Katrina & Waves, All night Long, 9 to 5, madonna (like a virgin or like a prayer), Tom Jones, sweet caroline, you can call me al, stevie wonder, Blues Brothers soundtrack, Commitments soundtrack, Think We’re alone now, Wanna Dance with somebody, Wake me up before you go-go, Only way is up, even YMCA on occation.
LOL and LOADS more really cheesy stuff like that thats all just off the top of my head.
Then add to it all of the pop music of the 90’s and early 2000’s: Shinia Twain, Robbie Williams, Anistasia, Amy Winehouse, take That, Boyzone, five, Backstreet boys, Girls Aloud, Sugar Babes, britney, christina, Belinda Carslile, Busted, McFly, george michael, Celin Dion, Mariah Carey (fuckin hate her!
) J-Lo, KT Tunstall, Kylie, james morrison, moderm Michael Jackson Stuff, mambo no. 5, Nelly Furtardo’s older stuff (I’m like a bird for example) Liberty X, Lilly Allen, Pink, Mark Ronson’s remixes (especially Valerie) Mmmm bop, etc etc etc
Then add to that a big pile of music for the young people
so thats lots of R&B & Hiphop and whatever’s currently in the charts: Lady Gaga, Pussy Cat Dolls, beyonce, Jay Z, Dre, Eminem, Destinys child, Rhianna, Black Eyed Peas, Chris Brown, Neyo, Usher, Akon, Jay Sean, Pitbull, Justin Timberlake, Timbaland, Nelly Furtardo’s newer stuff, flo Rida, kati perry, kelly clarkson, kevin rudolf, Saturdays, Shakira blah blah blah the list goes on and on. You could also thrown in a little cheesy dance here like the remix of Sex Bomb, Don’t call me Baby, sing it back, etc etc and use that as your building up the night.
That will be the main chunk of your night and will keep everyone of all ages at the party happy. Your probably talking about 40% worth of 60s/70’s/80’s, 30% worth of 90’s/2000’s and 30% really modern stuff for your average party, and you can just read the crowd and modify that accordingly, if its like a 40th or 50’s then obviously lean to the older stuff and if its full of sexy young girls for a 21st or an 18th then throw in plenty of extra “young people stuff” (although they’ll quite happily dance dance to all the old cheese as well!).
On top of all that you’ll also need a wee handfull of Rock in case the people at the party are particuluar fans and want some on, so a selection of stuff from the modern like Killers/Kings of leon etc etc all the way back through brit pop with Oasis/Blur/Charlatans etc etc, your 80’s guns & Roses, Brian Adams and Bon Jovi and right back to ACDC, thin lizzy, alice cooper etc etc of the 70’s so you have them just in case.
And also you’ll need some Chav (or neds as we call them in scotland) commercial dance music for similar reasons as backup incase someone REALLY wants them, Pretty Green Eyes, Bass Hunter, Cascada, and that ilk, older 90’s stuff like Ntrance, TTF, QFX, Underworld, and all the rave stuff, and then cheesy (read crap!) hard house covers of things like hungry eyes and celin Dion etc etc will keep them happy too. “bouncy” songs as they will be referred to here in scotland by the neds!
You could throw in some really commercial dance music if there are plenty of young people, like David Guetta, Bob Sinclair’s Love Generation, the Agnes song that was in the charts 6 months or so ago, La Roux, etc etc. Maybe play a few as your warming up people and building some atmosphere while they all arrive and get a drink and say hi to aunty betty who they havent seen in a year etc, it would mix it up and stuff, but to be honest I’ve never had a private party ask for normal decent EDM or house music, its always that cheesy hard chart shit or just pop music so i wouldn’t worry about that too much.
And then finally round it off with some classics from your area and only you’ll know what that’ll be. For example every single private party i’ve ever been to or the odd few i’ve DJ’d at as a favor here in scotland ALWAYS finishes with the same two songs, proclaimers 500 Miles first and then everyone makes a circle around the birthday boy/girl/engaged or married couple/retiree or whatever it is and they play Runrig’s Lock Lomond while everyone jumps about and the circle shrinks in to meet the centre people and then back out again in time with the music
When your drunk it makes SO much more sense, but it is actually quite uplifting and touching to be involved in it 
LOL, anyway hope that helps. I was gonna do another bit about what equipment to buy but this is already a big huge essay so get your head round that first and then we can work on what speakers etc to get 
k