I don't want to discourage you, but while the Cretors size engines are nice, they are also normally WAY out of your price range (move the decimal point of your spending money one space to the right, and that's without a boiler

). One of the things you'll learn is we always want more than we can grasp. This is a case where you'll probably have to modify your wants to fit your money
I would suggest you look on eBay for a good used Wilesco or Jensen "toy" steam engine to start (search for "live steam", "steam roller", "steam tractor", traction engine" or wasde through all the listings under "steam engine"). The medium sized ones and steam rollers aren't all that badly priced. I'd recommend a Jensen 25 (or 75) or a Wilesco D20 if you can find one. A Wilesco D365 steam roller can also often be had for about $100 used. They are usually dry fuel fired, simple to operate, and will last a long time if you take care of them.
Unfortunately ( because of the exchange rate), the prices for brand new Wilesco engines will be jumping about 10% on April 1st. The Jensens will be staying the same for now (US made and IMO a bargain), but look to spend about $230+ for a new one. . I'm a dealer for both brands, so I know these things.

(Unfortunately, the only used one I have at present is a nearly new Wilesco D6 which is a very small engine)
I haven't mentioned the more scale pre-machined "mechanic's" model kits because they are generally a few hundred dollars above your price range, especially since you want a boiler to go with them.
Be warned, though, once you start burning your fingers, you will soon find that you want some things to run with your steam engine. The various models start at about $20, but if you have an old erector set you can often build better stuff than you can buy.
Good Luck!