Well I am not a kit builder so take all this with a pinch [ bucket ] salt.
I cannot comment on the V5 issue , however I am sure someone will come along soon who can.
As for that car well it looks very rusty and very incomplete and in my mind expensive ?
Do you know how to use ' Completed items ' on Ebay ?
Look up Robin Hood I saw several that looked 75% or greater completed i.e body panels fitted matching wheels seats fitted etc and they were £1k to £1.5k then ones that had already been on the road for around £2k.
So you are not ' building' a car from a pile of bits however they all need some work and if you can drive it you can see what needs doing what can be improved and what is missing ?
Your £500 car looks cheap to you see there is no hood ? seats ? carpets ? radiator ? fuel tank ? only 1 front wing , no bonnet , one headlight ? rear lights ? indicators , wipers , windscreen ? etc etc etc
Also I would download the BIVA test book [ link above ] and see what you think to all the petty hoops you need to jump through to get one on the road ? ' E ' marked glass and lights and 2 speed wipers ? etc
Being tight buying and building is one thing IIRC the BIVA is £400+ and a retest is £90 , so that needs adding to the £500 RobinHood almost enough for buying a ready registered one and getting it right ?
Found this :-
Searching Google maybe productive ?
Thats why the kit marked has gone to re-panelling MX5's etc
Not dissing your idea , and get something and get it on the road I am sure the pleasure and grin is immense , however quite a few of the unfinished one's for sale are an unrealistic proposition to get through the BIVA.
If looking for a already registered one ask to see a picture of the V5c to see what it is described as ? Ford Cortina as a base car is fine however the V5c should NOT say that ? it is illegal to sell an incorrectly registered car however if you buy it and use it and are stopped you carry the can.
Does this shape appeal ?
Just find a nice Suzuki you get to build a new body from new parts so no rotting wood or rust/corroded panels AND you have built something.