Marklin and Stuart, you may be hard pressed to find much within your price range, and with the Stuart, you will almost certainly be short a boiler, unless you have a fair-sized stand-alone boiler already on hand. Of course, you can always use one of your Jensen boilers as a steam source for any orphan engines you may pick up, such that then you'd only need an adaptor fitting to replace your whistle.
Steam plants with everything you need included are quite common in the toy steam world, and many are easily found within that range.
Mamod, Fleischmann, Wilesco, Bing, Falk, Weeden and many others are out there and available to varying degrees.
Here is the link to the Toy Steam Bible, which is a website that lists very many of the known makers, past and present, and shows some of their products. Always a good reference in most cases, even though far from complete, it at least may help you get a handle on something you find that you don't already know about:
http://www.toysteambible.org.uk/default.htm
Good luck and have fun with your hunt!!! ;c)
I opened my budget range a bit.