Just to warn you, screw drive will be extremely slow. I built a screw drive machine and ended up buying a belt machine (mosaic, also RAMPS 1.4). My current x/y feedrate is 100mm/s, you'd need some incredible rpm to get that with a screw drive, even with a multi start lead screw. Before I had a printer I didn't realize just how long it takes to print anything sizeable.
I would judge print speed to be the most important machine decision, followed by x/y capacity, followed by rigidity, then vertical capacity. Accuracy actually doesn't factor in (in my opinion), as all but the very crudest machines can easily generate parts with better than a thousandth precision, which is under the noise floor of the plastic.
Do make sure to get a fine drive system on the Z, though, as you probably want to take your layer heights down below 0.1mm. That's where I'm at right now and I can probably go lower but for the huge increase in print time. 0.1mm isn't that bad for time as long as you only infill alternate layers. Most printers run around 0.25mm-0.5mm. For fast prints I step up to 0.25mm.
Garrett is also going to be building a printer, he's recently purchased a MendelMax.
I'd advise, when it comes time to select an extruder, to go with 1.75mm over 3mm mostly because Garrett and I have a decent stock of colors already. If you do go with 3mm, I have a small spool of blue you can have from my screw drive machine (which is going to become a CNC dremel).
My fingerprints are on Mars.
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