A few months back I was in exactly the OPs position looking at old road bikes on Gumtree. I eventually decided against it for the following reasons:
-Bikes up to the mid 1980s were much less standardised compared to today. Especially Ralieghs. You wouldn't want to get a 'bargain' frame and then find it has obsolete headset and bb threading, since those parts are likely to be close to going on an old bike anyway, it's very costly to find specialist custom parts today that would fit.
-As has been mentioned, both brakes and gears have come on leaps and bounds in the past few decades. Again it'd be a case of being either stuck with poorly performing bits or immediately shelling out for newer ones.
-I know friction shifters have their advocates but I think you should steer well clear if you're new to road bikes.
-Old wheels are liable to have the bearings shot, the nipples rusted, fatigued spokes etc.
It's not that you can't find good, well performing older bikes, it's more that you have to be very careful about what you choose and be prepared to spend more again for upgrades.
I would overall advise to save up and go for a Triban 3. I'd wager that it wipes the floor in every way compared to anything 'vintage'. Also if you're transitioning from mtb it'll be more reassuring with indexed gears and a threadless headset and the like.
In my search for a road bike I ended up buying a cheap flat bar hybrid frame and then bodging drop bars onto it. Good DIY option and lots of fettling practice but not something I could easily recommend.