Not sure where to start...?

I have played D&D since 1978, and just started looking into using a VTT like Roll20 (possibly as a player too), but it is confusing as to what content is available. Before I subscribe to Roll20 (or the other guys if I can't do it here), I want to make sure I have the tools I will need. I'm running a campaign that uses mostly just the core books, but I also use some UA content and I'm looking at adding the Xanthar's Guide into the campaign also. Is that possible with Roll20? If so, how?

A follow up question, is it possible to use just the tokens, maps and dice, and leave the rules to "off line" (i.e. as the DM I track it off line and don't worry about game mechanics in Roll20)?

I believe I may be able to help, Most books are available and do become available in a short period of time. I personally use the "offline" technique and have my own books while only using roll20 as a platform to create, roll, and so on. So to your second question, yes it is very possible. However there are many good character sheets that can also help with the game flow and allows for easier tracking for PC's Hope this helps.

Thanks Onyxcide, so where are the "books" if I choose that route, I didn't see them in the "market"...?

So, it depends on what you're looking for.  There are some DnD 5e books (Monster Manual, Volo's Guide), but not much more than that.  There are quite a few modules for DnD 5e, and more coming.  There will likely be more books coming as well.

Thanks Aaron, after walking through the tutorial I'm pretty set on playing, but I see a lot I need to prep before I can GM a game, the content is like paying again for the books (at full retail), so I'll probably run option b and use them "off line". The key I see now, is how to allow the characters to be automated as much as possible for players (the IT guy in me coming out). I don't want to have to enter it all manually, but paying $50 a book again is a bit much (I blame WoTC for that)

I recommend trying several games/sessions with your "offline" rules and, some automation set-up but not worrying too much about automating everything. You don't need rulebooks in Roll20 to roll a d20, or roll a d20+2, or roll 2d20. Then as DM, just like the tabletop, you say "You hit, roll damage" and they roll a d8 in Roll20. Many of the Character Sheets in Roll20 provide you with some level of automation without having rule books in Roll20. If you're playing 5E, the biggest time-saving Marketplace product is probably the Monster Manual, because this speeds-up your monster prep and drag-and-drop monsters. But you can set-up your own monsters without the MM, it just takes time and paying attention to small details in the configurations.

Thanks Gold, good advice. I may bite the bullet on the MM, but as you said running a few sessions without it first and run my Monster stats off-line also. Especially since I'll setup the maps ahead of time with tokens and all.

Go for it Richard. I run all my games without the online material like the compendium or MM (this is due to the fact I run osr games and there is no compendiums or drop/drag MM for it) and there is some initial prep work but not much more than prepping for a game at your dinner table.