If it's just for a trainer then I'd look at a spin bike instead; they take up less space, make less noise and will give you more stats.

If it's for using outside and on a trainer, then you also need to consider the trainer. Assuming it's a wheel-on trainer (the cheapest options, where you mount the axle and the tire sits on a roller) just requires the bike to have a quick release rear hub, which will have a lever on one side.

Other than that, just get any bike that fits you and seems to be in good mechanical condition.