Building it isnt hard but you will need help. You do need to make sure your foundation is fairly level though. Pick a day thats not windy and start from a corner. How are the joints on the ones your looking at sealed?
The prices you have got so far are great. My original plan was a concrete sectional garage as well and I had enough panels to make a single garage but wanted more and I was quoted £80 plus for each panel. I think it was a Marley one I had. Ended up giving it to a mate of mine, then had to help him build it.