I have an older model DR trimmer that i picked up at a yard sale a couple of years ago. It had been abused so i had to spend a few bucks to fix it up. Has the 5hp B&S engine. After i was sure it would be what i wanted, i ordered the new style trimmer head for it. Have been very pleased with it. Primarly used for heavy weeds and vines. I do see a lot of the Poulans returned to Walmart, not sure about the others you mentioned. By the way the new DR trimmers are high dollar.

I've got a Craftsman one that works great. It was I think $25 at a yardsale since they said it didn't work. I got it home and it started up with a little priming. I like using it much much more than my handheld trimmer. The line is so much thicker and lasts alot longer even compared to the .105 line in my handheld. I also don't get my legs/pants plastered with finely chopped debris.
I do see similiar items on craigslist so if you don't mind used, keep an eye out there on or ebay.

Yep, DR are a well made machine. They have some end of year sales here at the store in Southern NH? Don't know about your area. DR also sells factory refirbed units now.

Have a 20 year old DR trimmer and now use the 'knobby' line, which seems to cut the tough weeds so quickly. Per DR reps' suggestion, keep the line refrigerated! My only complaint is the inefficiency in oil changing. Not sure if new models have improved engineering. Would enjoy a narrow model to go betweern the raised beds as the hand held Stihl gets tiring. Have never seen one on CL.

Have used the Beaver blade without problems some years ago.