Have a look online for some reviews. I know this isn’t going to give you the entire picture as user reviews are often only written in two cases- 'product is amazing, or ‘product is awful’, but the % of either for the Gemini CDJs should give you an indication of where they stand.
Some of the knee-jerk reaction that sometimes pops up with Gemini is a bit unfair, but it is a direct consequence of some well below-par products in that particular market (CDJ250, CDJ600). The CDJ650 & CDJ750 are by all accounts a vast improvements on those models- I’ve personally only had a play on the CDJ-750 but I was very pleasantly surprised. However what these fleeting plays, and the official product reviews can’t show you is the long term reliability, and this seems to be the issue with the Gemini models.
There is nothing to say for certain that those Geminis won’t do exactly what you want, and I certainly wouldn’t tell you not to go out and buy them based on the name alone (I actually still have a Gemini DD turntable I brought new back in 2001 and it is still in perfect working order despite some heavy use over the years). However when you factor in that colour screen is going to be eating up a big chunk of the cost (the only other CDJs with colour screens are pushing $1.5 - 2k a piece!), it certainly suggests to me that some shortcuts have been taken somewhere.
There are models out there that, whilst doing away with the full colour screen and are a bit more expense, will get you a much more solid reputation. The models I would be looking at would be:
Reloop RMP 2.5 alpha
Reloop RMP 3 alpha
Stanton CMP 800
These are all solid CD/usb/midi players that have a considerably better rep than the Geminis. A good mate has had the RMP 3s for a couple of years now and they are superb decks.
However, the recent price drop on the Denon sc2900s that the poster above mentioned makes these an absolute no-brainer if you can stretch to them. They are full professional level decks, with the best jog wheel I’ve experienced on any CDJ full stop (and is tension adjustable) and they are built like tanks (a possible downside to this is that they are noticeably heavier than all of the alternative CDJs- but it means they don’t feel cheap & plasticy). I’ve had mine since they were released back in 2012 and they are real workhorses.
There are cheaper Denon units but they would only be available second-hand now, and many have a rotating platter (more akin to a turntable) rather than the normal static jog-wheel on the sc2900. Also most of the older Denon units have rubber play & cue buttons, a big criticism that was corrected with the hardened plastic buttons on the sc2900 & sc3900.
I would suggest that if you were willing to go the older, second-hand route then a better bet would be looking for a pair of Pioneer CDJ400s. These were the first generation of usb-capable players so the loading response might not be quite as swift as the newer players, but they would arguably be the best choice for you on the second-hand market. Solid units that have basic looping, and actually support HID control of software rather than just midi if you did decide to hook them up to Traktor. Here in the UK you can pick up a pair for around £500 (sometimes less), and you shouldn’t lose anything should you decide to sell in the future.
What you pay for the various models is probably dependent on where you are based, so it might not be quite so clear cut price-wise. But the thing to consider is that the Gemini 650 is arguably the most expensive of the ‘budget’ CD decks because of the colour screen (unnecessary by the sounds of your requirements). If you are going to take a chance on a pair like that you would probably be better suited going for something like the Numark NDX400s (or the NDX500 that has just been announced), as not only are they cheaper, but I would also be more trusting of their long term viability than I would with the Geminis.
At that budget level I would definitely be doing a LOT of research as I think the risk of getting stung is quite high- and these units will have minimal re-sale value. I would always advocate saving more and going with the proven stuff, however there are plenty of people who do use the budget stuff and they aren’t all breaking so you pay your money and take your chance. 