IF it has 1.5, then it should be very easy to go from a custom rom to the official 2.1, just by running the RUU program to get back to stock 1.5 and then doing the update (might not even have to RUU back to 1.5 since the update's OTW, but I'm pretty sure you do).
As for the preferred ROM, that's entirely up to you. Find one you like, with bugs you can live with (they've all got their different little bugs, but most are very minor), and start from there. Most people are either using DamageControl 2.08.1, or Fresh 2.x (whatever number Fresh is on now lol).
ROM's are completely personal preference. Some are already themed and a lot of people like that, others look stock but have other things working (like friendstream), and still others are pretty much just optimized versions of the official (or leaked) 2.1, and look and act completely stock (but you can them and customize them yourself).
If you run into a hiccup during the rooting process, then go back and read the guide again to see if you can figure out where you went wrong (likely just a typo, happens to all of us), and try again. If you can't figure it out, then ask here, and someone will help.