Personally, I'd recommend Optimus Prime (he's available for decent prices because of a reissue but he'll take off once again), Soundwave (because he's completely awesome) and Prowl (THE very best Transformer released ever) to start out. These three are iconic and the very top of the line.
I honestly can't agree with most those picks. MP-10 is a must own, but I think the Prowl mold is quite boring compared to the other MP cars and Soundeave is a huge disappointment. His hip design feels like it'll break at any moment, his alt mode is full of holes, the paint on his feet chip at the slightest provocation and Laserbeak is pretty much doomed to break given the size and fragility of his joints.
If I was starting out, I'd say get Prime because he's iconic and amazing. Get Starscream Ver 2 because he's the best MP Decepticon and an amazing version of the character. Finally pick up Wheeljack. He's the best looking and most fun of the MP cars. Maybe also get Bee as he's super impressive for his size and a ton of fun as well.