I have a Dunlap jack plane - no experience with it yet
Most of the Sears tools (and Wards and other department stores too) were made by someone else under contract. Major makers: Stanley, Millers Falls, Sargent, Union.
Walt Quadrato at Brass City Records and Tools has a handy guide for IDing bench planes made by these four makers, online at http://www.brasscityrecords.com/tool...lane%20id.html.
My Dunlap plane appears to have been made by Union, based on the lateral adjuster. Union had a pretty decent reputation.
As George Wilson said, you can probably (I would say, "almost certainly") fit another iron, if the existing iron turns out to get dull quickly; but you should try it out first. Often they're pretty decent steel. As an example, I have a Wards Master No. 4 plane (probably made by Sargent, based on the lateral adjuster), on which the iron is incredible - it went MONTHS of intermittent use before I had to sharpen it. If the cutting iron on yours is poor steel, you can shop around for a garage sale/flea market iron - I pick them up for $1 - or buy one from one of the specialty folks. Hock, Lie-Nielsen, and Lee Valley all make them. I like the Lee Valley irons because the back is machined nearly dead flat, to save you setup time.
On the other hand, I have a Sargent version of the Stanley No. 78 duplex rabbet that is clearly built to a price point lower than the Stanley - cheaper casting, no adjustment except your fingers for the iron, etc. I haven't had time to play with it, so I don't know how good the cutting iron is, but for $6 (missing a fence - it's going to cost me more than I paid for it to mount a fence, and I still don't know why I'm going to the extra effort...), it was a worthwhile buy.