Restoration project, help needed I've been gifted this frame by a family member and would love to restore it to its former glory. I literally don't know where to start. The brake levers are frozen solid and don't move at all. The gears are downtube. Obviously the first place to start is with some wheels - but Im not sure what cassette size I should be looking for. All in all any pointers or advice would be most welcome. Thanks!
The first question you should ask yourself is are you going to do this project is for fun and experience or for money? There is surely money to be made, otherwise the workshops like Steel Vintage Bikes from Berlin would not exist. 

As I can see you have been donated a (pardon my judgement) rather a generic frame and you won't be looking for substantial value of the finished product. Still in for fun and experience? Welcome to the enthusiasts world and read on.


Inspect and catalogue your item
Carefully inspect all the parts that you have, that are in reasonable condition for reuse and that you want to reuse
Decide what style you want the build to be done
It can be full restoration to original, you can improve/upgrade sticking to the era of the bike or go for full modification keeping the vintage frame and putting modern components (Campagnolo offers some groupsets in silver making them suitable for this retro-look)
Strip the frame
After disassembling all the parts look again what can be reused
Paint the frame if desired (or have it painted)
Depending on the condition of the paint of the frameset and the desired effect you can paint the frame yourself, have it painted or go for so called "rustoration" (clear coating the frame as is keeping paint chips and rust)
Collect all necessary parts
Some components can be reused, clean them, polish, grease, make them great again. The missing parts need to be acquired externally. Depending on the type of build you either have to source those second-hand (internet, local flea markets, dedicated events, etc.) or buy them new.
In my opinion the components such as:  


brake pads and brake cables should always be placed brand new (your own safety).  
derailleur cables should be placed new for reliability and convenience.  
bar tape goes new - hygienic and aesthetics. There is wide variety of retro and vintage style of the bar tapes available on the market, pick your favourite.  
for the leather, Brooks-style saddles, it's a lottery. It will either fit you or not. I haven't heard of successful refits of the leather.  

Assemble, adjust and enjoy


And that's pretty much it. If you have more questions, please ask. I'd be more than glad to share my experience in restoring bikes.