Lioness...Go for it! I think the i730 is the best phone available for both Nextel and Boost. I have not seen the 836 or 850 in use yet but the 730 is both durable and the features are very easy to use, I'm not a huge fan of the 860 or the 830 but if your looking for a handset that is in common use the 730 is the best.
As far as being locked or umlocked it kinda depends on where the handset was used..if it is not a domestic carrier you may have some issues but if it is just a regular Nextel it will work fine.
All you need is the Starter Kit and I can suggest a few locations for those if you need it...Jess