If it were me I'd go for a 55. Reason is, other than 10 gallon tanks one of the most often purchased is a 55. They always have good prices, and you often see them for sale used. You should be able to get the whole thing, minus a stand, for under $100. A 55 is not so heavy that it takes a special heavy duty stand. I have one on the couter top in my kitchen.....
I don't think your 10 is overcrowded, but I wouldn't add anything. If your guppies have many babies, it will be overcrowded.
Good Luck with the MTS, I hope you can find something you can afford.
