I wholeheartedly agree with all of the above but would add...
In general the best "tools" that you can buy are good references. Certainly until you are very familiar with these cars they will save tremendous amounts of time and frustration.
In the end, even if you end up spending a hundred or even two hundred bucks on them they will be the least expensive investment that you can make on your MG experience, no pun intended.
Get the Bentley manual first and then start accumulating other books, some of which will be more sub-system specific. Any of the Porter or Burgess books will be of value. The Des Hammill books published by Veloce Press will save your fanny some day too. Everything is available at some time or another on eBay or Amazon and can often be purchased as "used", which will save even more dollars.
Also, be sure to stick with at least one if not a couple of good boards like this one. The information exchange is great and everyone is always willing to jump in with their two cents.
Jack