Its a tricky one tbh.
Cheap ones on ebay are hopeless and will last a couple of thousand miles at best, and finding a good used turbo is a difficult feat.
A new k04 is around 800quid afaik
Your best options are probably either a frankenturbo or a scroll hybrid of some description.
Personally i dont think either options are worth it unless you can get it for a decent price (ie used in good nick). Paying 700quid or more for a new k04 series turbo is in my view a waste of money. I'd suggest you seriously weigh up going down the route of a T28/GT28R or larger. Might cost a little bit more than simply bolting in a k04, but its a far better solution for the money.
You can probably pick up a decent condition used T28 or GT28R from a nissan 200sx for around 300quid, and its good for a genuine 280-300hp. You'll then need a T25 flanged manifold, and custom pipework for the downpipe, charge pipes and oil/coolant lines.
The one advantage the K04 has, is that you can probably get away with bolting it in and not touching anything else, whereas a T28 is almost certainly going to need a remap to suit at the very least.