Chainsaw Repair

It's confusing to me about the numbering and I'd. Of a Stihl saw. Like a 026, I have read where some guys love them. Saw a 025 today, they seem small and not a large cc. I am cutting 3 large sweet gums down and looking for a Stihl chainsaw at a pawn shop to fulfill my needs. First tree is 49-50" diameter at the bottom. Others are smaller somewhat. Cut the first tree with my Timber-master, Poulan 3300 with 20" bar. Still lack the bottom 6' cutting up small enough for the city pick up guys. Wanting a saw with more umpph. Probably a little longer bar also. Would you all recommend what size Stihl would be suitable and please explain the numbering of these saws some. Like if you have a certain no.saw how do you know like the age of the saw ( yr.) I found 4-5 Husqvarnas today. They were either 450 or 455 Ranchers today at a pawn shop. Not sure if they are bigger and big enough for me to go ahead with my cutting. Got my Poulan here in my town at a Poulan factory that was reconditioned probably10-15 yrs. ago.The local saw shop I go to says it was a ok Poulan. That mine was not a Lowes quality Poulan. Anyway I'm looking for a Stihl, probably the last one I will ever buy. I take good care of them with maintanence. Can I buy one in good shape? Or **** poor shape at the pawn shops. Put an ad in Craigslist today looking for a 25"-26" bar, 50-70cc. Preferably the larger cc.  What identication no. on the Stihl be for the size I am asking about. What's the gas/oil mixture ratio for the Stihl saws and Are the Chains anymore expensive for a Stihl. Do I buy a certain name brand chain? I know I have seen and bought a Stihl brand chain before for my father-on-laws smaller Stihl. Please educate me, I want to make the best decision for my purpose. Again explain the numbering on these Stihl saws. Thanks

Cut4fun thanks for the info. I really do not know my own saw very well. I never realized my saw was 54cc.
 I think I will go to my local saw shop this morning and buy new fuel lines, fuel filter and replace or thoroughly clean my air filter. I did a little more research it says my saw was introduced in 1987. It's not as small as I thought it was. I assumed 3300 means 33cc. See what I mean when asking about the nomenclature of saws. If I have never had a Stihl or maybe even a Husqvarna then these no.s are very misleading. Like 026, I assume something like 26cc. Thanks for making me think more about what is actually in front of me. Things are not as they appear sometimes.
Will clean my saw up again and service it and re- attack that tree butt again. Thanks for your reply.

RLS -

Where are you located?  That would help us a bit in recommending a saw for you; I'm thinking we could help you find some pretty good ones on your local Craigslist.

Bottom line - you should be able to find a nice used Stihl pro saw in 50-70 cc without venturing too far from home.

Scott