You definitely need to be putting cash in every time you fly, because that way you fund maintenance as it arrives, and whilst it isn't in your spreadsheet so far, you need to add a little extra to build an engine fund. With only 400 hours left, you'll need a new one or zero timing in 4 years or less (presumes stays healthy), and you'll need to budget at least £20,000 for that. Over 400 hours, the necessary premium would be very high hourly costs. So it may be that is when you need new group members to buy in for the new engine - but it's likely you will need to spend the cash up front before you can attract them! But get into the habit of contributing the extra to the engine fund from the start. You should budget at least £10 per hour if not more. Then when your new engine reaches 2000 hours, you have at least £20k in reserve.
On airfield choice, if you are buying from Earls Colne, and they have TB maintenance there, it would seem a good choice, with hard runway that's easily long enough, and nice people there!
Drifting slightly, there is a TB languishing at Rochester at present. It hasn't moved for year, so would probably be for sale at a reasonable price, but would need lots of TLC, but maybe you could do it up with just what you want, paint, radios etc.........just a thought. Engine life remaining is always very important. Good luck.