From all the information I can find the best cars to convert are older rear wheel drive cars that are light and have enough space to house batteries.
Older cars have less components that would need re-engineering to be compatible to your new EV conversion, for example a simple car with no power steering, Air Con, Cruise, Sat Nav, etc. Although many of these things like power steering can be converted quite cheaply with used electric power steering pumps from Vauxhalls for example.
It tends to be there are more conversion "kits" out there for older cars, mainly older VW, Porsche and Mini. I say the word kits loosely, as from what I have found there isn't a kit that encompasses every single part you need right down to the last wire. However, I'm sure eventually this will be a reality, bringing more people on board as it will require less engineering know how to complete a conversion. But expect a full kit plug and play for a generic car to be £15,000.
Light cars will have an advantage as a smaller motor and less batteries will be needed for a decent performance and range, which will also keep the cost down.