Damyns Hall is actually closer to 800m long on runway 21/03 (no matter what they say in the charts). 21 is relatively clear too. There is however a gradient down 21 (and up 03). If you were fully loaded and the wind favoured 03 for departure by more than around 5kts (if below takeoff 21 anyway) but less than around 15kts, you wouldn't be able to safely takeoff (well you'll get airborne, probably, but the big pylons and cables on the extended centreline might not greet you softly... And I don't think you'd out-climb them fully loaded in a TB10. Runway 14/32 wouldn't work for a TB10 as it is a bit short - fine for landing, but too short for takeoff (and is very bumpy at the moment). Otherwise, there isn't much of an issue. Thurrock for maintenance is only a 5min flight (very good reputation there), AVGAS is available, but not all day every day, so might need a bit of fore-planning; not possible to land at night, nice bunch of people and during the summer the cafe (or weekends all year) is pretty good. Also - no landing fees which is pretty awesome! [things to bear in mind for your choosing].
Yes, I would recommend you put all the static costs together and divide them by the number of people who have equal shares, and pay them either monthly or annually into the plane's bank account. Then charge yourselves the variable costs as outlined per hour. If there are 4, I'd hazard a guess at £220pm (ish) + £100ph wet. In that case - if she doesn't fly, or some people in the group fly less than others, its still fair as they own part of the plane, and these static costs don't change whether or not the planes are used. Also remember that planes like to be flown, not left to rot, so if someone pays £200+pm, they'd want to get their worth out of it!
Don't estimate the number of hours you think she will fly to base your costs on as if she flew less hours than anticipated (which is quite common) you'd be left out of pocket...
If you wanted to rent it out as well though, ie: to allow yourself to top up the account, and help pay for that £25k overhaul (well might be more, might be less... you never know!) you'd probably want a "membership fee" of something reasonable like most clubs, say £200pa and then charge £180ph wet or something like that - but you might not get many takers, and would ruin your availability probably!
Anyway - things to consider...!