1) Where should I buy MOTU toys?

I would suggest Ebay because there is so many MOTU toys listed everyday.

2) Do I buy them loose, carded or AFAed?

I think that you need to figure out what kind of MOTU toys you want to collect. As a new collector, I would collect either loose or carded stuff. I would DEFINITELY stay away from AFA/Graded. Why? Because I seen so many collectors get wrap up in the value and the money of it all. Collecting MOTU should be fun ! I personally started collecting low condition MOTU on cards. When I had all the USA collection on cards, I then upgraded to having all of them on mint cards. When I had all USA on mint cards, I started collecting variants, foreign cards, etc. The main thing is concentrating on what is fun.

3) How much is MOTU toys worth and how much should I spend?

I think that question is best left up to you to decide. When I was a kid, I value a He-Man 8-back as $3.99. When I first collected MOTU as an adult, I value it at $100. Today certain pieces I think is way undervalue and certain pieces is way overvalue. I think it comes down to 2 things to value the toy you want, how much is the market value and how much do I value it? I once paid at the time the highest price for any MOTU toy eversold on Ebay. Did I over paid? No, I actually I felt I underpaid because what I paid for the giftset was only half of my actual bid. I value it alot higher than most people because there was 2-3 pieces that will complete my USA collection. The value for anything is what you value it at and not what anyone says.