Short answer: get it now
Many of the older Ultimas are very dated and, despite many great qualities, difficult to get into if you didn't play them when they were new. Ultima VII is the first in the series to exclusively use a graphical, mouse based interface, and it is still a highly playable, (mostly) intuitive game.
It is also an incredibly deep RPG with a rich, colorful world where you can walk into an empty house while the inhabitant is at his scheduled job, make dough, knead it, bake it into bread, spin wool into thread, sew handkerchiefs, cut them into bandages, steal the silverware and candlesticks, and throw the fork at someone as you leave, as long as you don't let your nobler party members witness what you are doing.
The conversations are intricate, the cities are large, the puzzles are challenging, and there are a wealth of secrets to find if you look carefully.It's everything you could want in an RPG.
While the story builds on earlier games in the series, it is the beginning of a new trilogy, and it mostly stands alone. Read through the documentation or a wiki entry to get the general gist of the past games, especially to learn about your party members, if you can. Some of the optional quests really build on older stories, but the main game works well even if you don't know much about the series.
I highly recommend using Exult to play the game. It can be found at
http://exult.sourceforge.net/
I would assume that GOG's version runs on dosbox. Exult uses the original assets and recreates the game from them with a substantial number of enhancements. You can also get high quality .ogg files sampled from the intended sound cards of the day as well as a few mods that add content that was cut and smooth the gameplay further.