Have you, or any of your group actually sat in a TB10? I had my eye on one recently, until I sat in it. I'm 6'4", and my head was hitting the top of the cabin, without a headset on. I couldn't see a height adjuster on either of the front seats. I'm led to believe that it's caused by the main wing spar passing right under the front seats. Worth a thought, if you have any tall group members.
On the cost side of it, until a couple of years ago, I was part of a group of 3 with a Grumman AA5. We all paid £100 a month, and £2.00 a minute airborne time. This was enough to cover our ongoing fixed costs, occasional maintenance and fuel. Come annual time there was usually something left in the kitty, and we 'cash called' for the balance. We actually sold it, as it was vastly under used in the last couple of years, and the cash calls became too much.
As others have said on here, I think your estimate for your annuals is a bit light - double it!
Also be aware if you are taking it away from the current maintenance organisation, as a new organisation, unfamiliar with the particular airframe will likely be - quite properly, as they have to sign it off - more 'picky' on marginal maintenance issues which the previous maintainers would have been monitoring. Expensive and demorilising experience for the guy who bought our aircraft, and moved it to a different shop.