DO THE RESEARCH FIRST.
I can't stress that enough. Check over all the armies you can play. Double check. Read up about any that have you interest in. Cool fluff? Cool models? Check out all of those armies that tick those boxes for you.
Still set on Tau? Welcome to the Greater Good.
A few things about collecting. This is a hobby as much as it is a game. There's no such thing as having "too many models". It'll be OK for you to collect multiple models. At times you may not use all of them, but it's nice to have a large selection to chose from at the end of the day. First thing first, what do you plan to do with your army? What is your goal? Figure that out first. Once you do, start putting together lists that you would want to run with that in mind. The last thing you want to do is to start collecting, assembling, and painting an army that does not do what you think it does. It'll feel real bad.
Lastly, once all of that is done and you know what you want and have the right expectations, you may start collecting. At this point you'll be trying to hunt for good deals to help you save money while you build your collection. The start collecting boxes are an excellent way to do this. There is a larger bundle coming out later this month that you may want to pick up as well that could save you some cash in the long run. http://www.belloflostsouls.net/2016/11/ ... -sets.html
Might be worth it to look around at other websites too. This is the easier part of the process but requires some patience too. If you have plenty of disposable income it won't be too difficult, but saving money feels great.
As far as painting goes, you'll probably find a wealth of knowledge on these boards and there's plenty of videos on youtube too. GLHF.