doanh
Welcome to TF!
What did you need to know? The ST series was made from 84-95. The ST120 you're looking at is probably from 86-87? Should be 12HP B&S.
If you go to
www.sonnysbolens.com and click on manuals at the top of the page you will be taken to ST series and from there you can download a parts manual.
We have another member with an ST160. His name is boB. You may try emailing him for some first hand info. He posted about his tractor in the "what model do you own?" poll.
Most important questions are what is the overall condition of the tractor? Will it need any repairs before it can be used? How big is the deck? Is the 1.5 acres flat, hilly, all grass, partially wooded, etc?
Chances are the deck is 42" and should be able to handle 1.5 acres. The tractor in good condition is definately up to the task, but deck size will determine how long it will take to mow.
You haven't mentioned the price. It might be easier to offer an opinion knowing what else is available in the price range. If you have any concerns about parts, contact someone in the parts supplier thread, such as Ray's Mower and inquire.