My 2p...

If you're actually going to drive it, low mileage isn't great IMO as seals dry out, and things go wrong with cars that stand about - cars respond best to regular usage. Plus there's the issue of destroying the value by putting miles on it. Also, a GP isn't that different from an R53 to warrant the premium if you're actually going to drive it - the money saved buys a lot of mods...

Also, don't be sure that just because cars are up for sale at a price, that they're actually worth that and selling for that. There's a lot of people being sucked along on the whole modern classic/investment bandwagon and it's just nonsense. You need to find out what cars are actually selling for, and I bet it's a lot less than the advertised prices. Brexit etc, and the natural cooling of the market after the rapid growth of 2014/15 means a lot of sellers are both delusional, and unwilling to accept a realistic value or take a loss. You only need to look at someone like Hexagon to see inflated prices that cars will not sell for. E39s being one example of some prices just getting silly.

If you really want a GP then great, but study the market to see what actually sells at what price if possible. The market is flat (for investment/collector cars) and it's the wrong time of year - but a lot of sellers are refusing to drop their price to acknowledge the new reality. I thought about selling a car this summer, and the offers to buy were substantially less than what similar cars were for sale for.

Just keep all that in mind - there's a lot of people that think 'my car is worth x' which is fair enough when it comes to guaranteed value for your insurance; but your car is only worth what someone is willing to actually buy it for on that day.