Need opinion on upgrading groupset for my road bike I just moved to UK for my masters and was looking for a road bike. I found a pretty old ribble 631 road bike taht was barely used. Its supposed to be around 20 years old but still has decals intact. The seller said that the bike was too large for him so he just used it on a turbo trainer. I paid £560 for it.
Rear derailleur - Shimano 105
Front derailleur - Shimano 600ex brand on 
Brakes - Shimano 105
Brake levers - Shimano 105
Gear levers - Shimano index RX400
Rear cassette - shimano 12 - 24 ( 7 speed )
Wheels - Mavic MA3 rims on Shimano 105 hubs
Headset - Shimano 105
Stem - SR quill 
Bars - Richey Pro T6
Seat post - Titanium with alloy layback clamp
Seat - San Marco Ponza
Tyres - Michelin axial sport
frame - ribble 631 reynolds road.

Although the bike is in pretty pristine condition, I am not used to downtube shifters.
The shifters are downtube, the front is friction, the rear is indexed. . I am thinking, I'd upgrade parts of the groupset, to a 10 speed cassette, bar end shifters and rear derailleur. The cost comes to another £250 if I source the items as cheaply as possible.
Here are my concerns -

Did I overpay for the bicycle ? I do have a feeling that I did :(
Should I spend the extra on the upgrades?
That's a nice enough bike, but it's not something to upgrade, you bought the wrong bike. Either you want a vintage bike in which case it sounds great, or you just want a road bike, in which case something with Shimano Claris R2000 would be suitable.
Assuming you have 400EX, then this dates from 1990-1992. 600EX would be pre 1988, so it's possible you actually have Exage Sport, which is more contemporary with 600EX.
631 tubing however suggests more 1990s than 1980s
I would suggest a value maybe £200? https://www.facebook.com/Retrobicyclesrecycle/posts/3754668234607817
There is nothing special about those components, though they aren't bad.
I had a look on fleabay and you could buy a generic modern(ish) bike such as a Triban 540 with 2x10 speed 105 and carbon fork for about £300. Or you could buy a new 2 x 8 speed bike such as the RC 120 for £400.
Not much to do with it except buy some new tyres if they've perished. Sell it if you don't like it. Buy a more suitable bike.