I know its been asked but I can't find exactly what I want so I ask.

The local small engine repair shop will sell me small engines for $10 a piece. I looked through the bin and found lots of stuff. I need to know which ones to buy and which ones to stay away from. There are lots to choose from.

Ryobi weedie 31cc
Ryobi blower 31cc
Craftsman weedie 25cc and 28cc
Troy weedei 25cc
Echo hedge trimmer 25cc
Huski weedie ?cc
Homelite weedie ?cc

Some have bolt on muffler some don't.
Some have High and Low adjusters, some don't.

I am just starting in to conversions because of the larger planes that I have reciently aquired. And I love to tinker. Will be learning all about prop adapters and lightening engines.
Which ones are gold and which ones are lead?
Also what is the rule of thumb for engine size to plane size? I have everything from 0.15 glow planes to an 11ft Zero so I need some of everything.

Thanks in advance.