I'd agree with Swee'pea
I'm riding a 1980 Holdsworth - had the frame from new, along with same vintage Brooks saddle, that apart not much is original.
But really enjoy fettling, so am constantly tinkering and buying good/NOS old parts. The BB is a modern sealed bearing, the brakes are new, the wheels are old 105 hubs 700C with 7 speed block, but in good nick. Gears are traditional downtube shifters with Suntour rear & Campag front.
It's not a purist’s bike, but it has character and is a joy to ride.
The point I’m making is don’t expect to pick up a sub £100, 20/30 year old bike and expect it to be any good unless you’re very lucky. It may have the makings of something special, but you’ll have to spend many hours getting it right.
I personally have had very few mechanicals with my set up, it's reliable and it's never been in a LBS for a service or repair. I've done some Audaxing, regular 50 - 100+ mile rides and several fnrttc.
It's likely that STI's etc would be more sensible and efficient, but they are technically complex (not to mention expensive) and I doubt I'd be able to easily and cheaply resolve problems arising.
So you have to be clear what direction you want to go - how much risk you want to take and how much you can DIY, otherwise it may eat up your time and ££££! It's great fun though.
