To root, I would recommend the app z4root. You dont necessarily need to flash a ROM to get the hotspot, but if you want to flash a ROM, I would go with liberty. It all depends on what you want your phone to do.
I would recommend reading up on rooting and ROMS and such before doing it.
Rubix is by far the fastest of the 3. Fairly customizable, and well supported.
Fission is the middle ground. Out of the box it isn't much different than stock w/o motoblur, but it has loads of customization thru fission rom manager. Not as many options as rubix tho
Liberty is the most customizable of the 3, 2nd fastest to rubix, but has the best battery life. It is also the most stable in my opinion. Liberty has their toolbox (like fission rom manager) with everything from extra apps, themes, etc. to customization options of the pulldown bar, and other framework changes.
Overall, you need to find what you like, and what fits you best.
If it were my, my choice would be
But it should be trial and error until you find one you like
As TB said, z4 is the easiest rooting option. If you do decide to go custom, you'll need to install clockwork recovery using rom manager from the market, and you'll need to get the droid 2 dootstrapper. I have a copy of z4 and the bootstrapper ready for distribution if you want it. Shoot me a pm if you do
From my Liberated Droid 2 (Liberty v 1.5)
you have to root. Other than that you would have to use the pre-installed app for $20 a month, believe prefer there phone rooted thancurrently in a warranty replacement fiasco but if you want to flash a ROM, I would go with liberty.