You don't need to unlock it to get more "apps" (man I hate that word). Everything you can install on a custom rom can also be used on a stock Sprint rom. Granted, the custom ones come with the latest WM builds, packages, and of course HTC Sense.
The unlocker is restricted to one use per computer station and it is verified online. You can easily "donate" online to avoid this mess. There have been some strange issues with the unlocker lately and I'm not sure if it's still functional or not. You can always try to see if it works.. you don't need to flash a custom rom after that. If it fails, just reset the phone and no harm done (not to say it won't work).
WM6.1 is thought to be more stable and quick whereas 6.5 offers new features and arguably better battery management. 6.1 is relatively "old looking" and stripped when compared to the later releases, in addition to being less finger friendly IMO and of course not having the cool scrolling of 6.5. It rocks as far as memory usage and stability however.
Bricks are very rare and even when they do happen, they're recoverable in the vast majority of cases (so they're not REALLY bricks are they?). People think they brick their phone when they flash a GSM rom on a CDMA device. In fact, the phone can easily be brought back to full function. Now flashing a GSM radio is a different issue..
My advice is to read the sticky threads, browse the upgrade forum (pay particular attention the the first posts of threads made by chefs), conduct a few related searches, and ask specific questions in this forum.
I suggest you ignore "radio unlocks" for now. Stick to unlocking the device for the ability to flash a custom rom. For that, go here:
http://forum.ppcgeeks.com/cdma-tp2-u...w-hardspl.html - Then select a rom to download and flash in the upgrade forum:
http://forum.ppcgeeks.com/cdma-tp2-upgrades/ - you can sort by number of views/replies and start with the more popular ones to get a feel for it. You can flash as many roms as you want (well, technically I think the limit is ~10,000). The optional step is to use task29 to wipe the phone totally clean before flashing a custom rom. This is NOT needed when flashing an official Sprint rom! I've attached the file in the past, look through my posts to download and learn to use it.
Be advised that the majority of roms use 6.5 with the newest version of HTC Sense, a combination which many people will tell you is far from the fastest. If you want absolute speed, look into the base 6.1 rom, but don't foget that you'll have to install a lot of your own cabs, tweaks, registry edits etc.. I don't recommend this if this is your first time doing it.
Post questions here and you'll find people who will happily help you over the bumps. Just do us all a favor and read a little first because some of the questions get pretty old.