I've sort of been there done that. the sensible option would be to sell your g40 and buy one fully modded up with the extra money, it might save you if you plan to go down that route.
when faced with the same dilema I bought another G and it is crippling, I wish I kept one of them though, I guess the wrong one sold first.
Also depends on whether it is your main car or your sendond, third or even forth car.
at one point I had the golf, 2 g40's 2 mk3 saloons and my g/f's 9n. now I've sold the 2 g40's things are a bit easier, stoopid me though picking up a loop tommorow

i never learn.
personally in your situation I would forget about the other g40, dont you already have a golf and a gt? spen whatever you need to to get one of them up and running, then sell it (creating more space) then the money you get from that spend on your G! or a nice holiday or something.