M18 FUEL 18-Volt Lithium-Ion Brushless Cordless String Trimmer with QUIK-LOK Attachment Capability and 8.0 Ah Battery

Milwaukee 42-26-2718 Edge Blade search that part number and that should give you a good lead.

This is just the saw attachment. You need a Milkwaukee power head (String trimmer head) for it to attach to.

This unit is designed to accept tools created by Milwaukee for the power head. I have seen no indication that Milwaukee has any kind of blade head. So I doubt you could do this without perhaps significant modifications that you have to make yourself. It is a great string trimmer. Trims grass well, and has great battery capacity.

Yes. It will cut thru weeds and brush up to 1/2 inch. The trimmer is not intended for brush but will cut. It is a beast.

I have the wedge trimmer attachment and have used it quite a bit. I do use it sometimes for cutting light brush. But it needs to be swung slower than a regular trimmer with a cutting disc blade. You need to work a bit slower and with more thought when using it. Bare in mind it's a clipper bar and the weeds needs to get in between the cutting teeth to be cut. Once you slow down it does a fine job. This tool is a bit awkward to use without a strap and honesty the strap that comes with it doesn't work very well. After I used it a while I purchased an inexpensive brush cutter harness and that improved ease of handling substantially. The main purpose of this tool is for cutting hedges of course and for that it shines. Personally I wouldn't want to be without my regular length M18 hedge clipper OR this M18 hedge clipper attachment.

About a 3" plastic wheel that can be adjusted in height.

I think you mean, "Can one use a non-Milwaukee Quik-Lok head on the string trimmer attachment?" An important question to address before this one is, "Is there a non-OEM head available?" I'm not aware of such an availability, so I think the answer to the question you posed is, "No." That said, I'd like to ask why would you want to change out the head with one by another manufacturer? String trimmer heads are problem enough without adding a possible incompatibility. If a replacement OEM head is not available, I would call Milwaukee's customer service department and ask for help.

The one for 329$ doesn't accept different attachments like the polesaw and hedge trimmer attachments. But it does come with a bigger battery. I think the one for 299$ is a better deal.