I would recommend a blueprinting/restoration project. It can move into a modification and then with the same parts on to your own design.It would need good quality parts, and be versatile. There are a fair bit of choices. I'll mention some of the ones which stick out for me.
A Citation II would stretch your budget a bit, a pair of Heathkit W5's would land you closer to the top by the time you have restored them. A pair of Dynaco Mk.IV would allow a bit of budget to try other things when the modification starts to happen. Eico HF 50's are going to land yo near the top of your budget limit, but have good Iron and thus potential...
regards,
Douglas