You've probably made the right decision to buy rather than build. Firstly because building a bench isn't complicated but it's a heavy job if you're single handed (I know I've done it more than once) and it's certainly not cheap once the wood and fittings are costed. Secondly because there are some stunning used bargains out there, providing you can collect.
But you'll need to be patient and you'll need to be aware of the pitfalls,
1. There's a misguided view that old record vices go on forever. They don't. They rack, sag, the quick release wears out, and they suffer from installation problems. Tail vices are even worse! So don't just assume the vices will be okay, once a beefy fitter with a tommy bar extension has mis-used a vice to press fit a bearing it's unlikely to ever be the same again.
2. Tops get chewed up. Not a problem in itself if you can resurface it, but beware the bench that's twisted or thick with grease and swarf, and generally looks like it's been used for metalwork.
A good workbench makes woodworking much more efficient and satisfying, fine examples are out there at cracking prices, but you'll have to persevere to find one. Good luck.