Well, if you do decide to buy JA, you'll be happy to know that you install mods the same way you do for JO. Here's a short list that explains the process:
1. Download the desired file to any directory you want, particularly one that's quick and easy to access (I usually put mine in a special mod folder under "My Documents").
2. Open the file. Most mods are put into a zip file, so you'll need WinZip or a similar program to open them. You can download a trial version of WinZip or buy the full version here
(just download the trial though, it has everything you need. Just keep selecting "Close" whenever they want you to register, as there's no time limit for how long you want the "trial" period to last
3. Once you have opened the file in WinZip, be sure to open the README file that is (or should be) included and follow its instructions. If it's a skin or a character model, you'll most likely extract it to JO/JA's base folder. If it's a full blown mod, you'll want to put it in a new folder under Gamedata. If in doubt though, ALWAYS check the README file.
4. Fire up the game and have at it (NOTE: for mods placed in a new folder under Gamedata, you'll need to activate the mod from the menu by selecting "Mods" and opening the folder you put it under).
I hope that helps. As for whether JA is worth buying, I would suggest checking out this thread