I did something similar to Monocock with my first aircraft, a C150 Aerobat. Scruffy with ancient radios it was a good blank canvas. I had her stripped and sprayed, a top overhaul done on the engine, new radios, nav aids and a leather interior. I'd have her still if 90mph winds hadn't turned her into a heap of scrap ally
Get an engineer through recommendation, arrange for him to go over the aeroplanes you see with a fine tooth comb. If the owner doesn't like this approach put your chequebook back in your pocket and walk away no matter how good the machine looks.
I'd agree with keeping a minimum of 20%, preferably 30% of your budget to cover the unforseen snags that always seem to arise in the first few years of ownership.
Regarding the simple / complex debate, try a Reims Rocket. It's a C172 with a VP prop and a 6 cylinder 210 hp Continental. Not as fast as a retractable Arrow but you have 2 doors, excellent field performance, a reasonable cruise, superb load lifting capabilites and you can get a good one for around 40k (pounds that is).