It really depends on your budget. Indoor trainers are very definitely a place where spending more money makes a huge difference.
You need to consider what you will run your training software on (if any - you don't need to, I use Zwift, albeit infrequently during summer), fans and cooling, which trainer you want, and the bike you will use it with.
I'd suggest budgeting a minimum of £50 for a decent fan, if you've got a good smartphone or laptop then you can use that for running Zwift or Trainerroad or whatever training software you want, and if you have a bike then just a few checks to make sure it's compatible with the trainer you are considering.
In terms of the actual trainer, you need to consider how often you want to use it for and whether you want to use it for just training or going for a ride in a virtual world such as Zwift - you mention this in your post so I'm going to proceed on that basis. You need a smart-interactive trainer. You can use a dumb-trainer but you will lose the interactivity of the software which is what makes it IMO actually fun to use/not dangerously tedious depending on how you get on. An interactive wheel-on smart trainer has the lowest cost of entry, but depending on how powerful a rider you are can be a little on the noisy side. The sub-£600 mark is hard to get started in though for a proper interactive trainer - the Tacx Flux S is a good unit I've used one myself - and for half the price of that you get the Tacx Flow T2240 which is a cracking unit in terms of getting started and quite a few members here use one, but if you're a more powerful rider you quickly run into the relatively low power ceiling that the unit has. That being said it's also about a quarter the weight of the Flux S and it folds away nicely underneath a bed or into cupboard which the direct drive units fail to be able to do.
You also need to ensure that you have the space to set one up and use it, so bear that in mind also.
A lot to think of though.
