1) Power of the radio (15 mw vs. 100 mw) is the only major difference in the two model families. Longer distance, higher power would be advisable, so 350.
1 again) There is a Bridge Antenna Calculation utility at http://www.cisco.com/warp/public/102/us-calc.xls that does these types of calculations. Remember to consider the Fresnel zone in any design--Radio line of sight and Visual line of sight are two entirely different things. Your Cisco Wireless reseller should always survey the site to ensure that the site is appropriate for wireless.
2) Yes it's possible, but not pretty:
Bridge A (root) to Bridge B1 (non-root) on SSID1
LAN at Site B
Bridge B2 (root) to Bridge C (non-root) on SSID2
3) Spanning Tree only ever has one root, and so does Cisco Aironet Wireless (unless you change SSIDs). The Root bridge manages associations and such for the group. If logical data flow dictates that A is at the center of things, then yes, Bridge A would be the most appropriate candidate to be the Root.
Actually the BR-340 is 100mw, so either bridge will be fine for the distances you stated. As for chaining the sites together .. I've done this quite a few times and it's no problem what so ever! The biggest issue is placement of the antennas (using vertical seperation and RF contour nulling). Each BR-340 (or 350) can be linked be a hub or crossover cable (if you do not need local access at that site).
You can use a variety of configurations to make this happen: all wireless- Using root bridge and non-root config desc. above. Or doing a direct or hubbed connection to provide dedicated bandwidth between the far end points.
As noted above LINE OF SITE is mandatory (incl. Fresnel zone calculations)