As with so many other things a lot of it comes down to how confident you are with fiddling with stuff and how much more time and/or money you're willing to throw at something you buy used.

I recently bought a clock for restoration with a particularly small and fiddly escapement movement. I bought it to learn about the particular type of movement, knowing there's a significant chance that when I take it apart to get it working again something is going to fly across the room and the thing will never work again. I restrict my purchase prices to take that sort of thing into account - if some tiny component pings across the room and I can't find it the rost of the clock is a collection of spare parts, unless I can source replacement parts (which is hard, if I don't know just what it was that pinged). Hence I might pay £10-20 for a clock that needs attention but if a clock is listed at £100-200 I'll leave it, simply because I don't have the expertise to be willing to risk that extra money on an unknown.

Likewise with bike parts if I bought a wheelset for £50 knowing they needed attention I could spend it figuring I'd either fix them and either use them or flip them, or the worst case is they go in the trash and I salvage the hubs and maybe some spokes and rims. If that wheelset cost £500 I'd give it a miss, for me it's not worth the risk. Someone who is familiar with stripping and servicing hubs might be more willing to take the chance because they would be less likely to end up taking the whole lot to a bike shop and paying professional rates to get it fixed.