Hi Nick! Glad to see another P3Steel soon to join the ranks.
Megapost ahead. 
I sourced my own parts, mainly to customize it for my own purposes. I already had a direct drive single extruder Prusa i3, and wanted a bowden dual extruder version as well as 8mm leadscrew driven Z axes. If you’re fine with the kit’s options and can’t save much from sourcing your own parts, I don’t see any reason you shouldn’t use the kit.
I wouldn’t suggest welding the frame together. You may need to disassemble some parts of the frame in the future for tweaks, upgrades, or replacements, and honestly, the default frame is more than solid enough. It’s already cut in a pattern where the frame elements slot into each other for extremely rigid 90 degree angles. I don’t feel uncomfortable at all standing on it while it’s printing.
As for extruders, my MK7 on my Prusa i3 and my MK8-based extruders on my P3Steel have both been quite reliable and have produced excellent prints (though keep in mind the MK8s are mounted behind the “shoulders” and not on the carriage, so the stresses may be different), and the MK9 appears to be the same basic design with minor differences. It will probably work well. The Bulldog extruder does look to be a better design in terms of torque and precision, but the two issues I see with it are availability and the added weight of a gearbox (the X carriage would be more limited in its speed, assuming you’re using a direct drive extruder). Also, you may have to design custom mounts for the extruder on the carriage or anywhere else if you’re doing a bowden setup. If you’re willing to deal with all of that the Bulldog could be a worthwhile upgrade.
EDIT: I should mention that you’ll likely experience a failure with extrusion at some point during the printer’s life, but more than likely it will come from a jam either inside the heatbreak or in the nozzle rather than the extruder. Because of that, it’s pretty important to have a few spare nozzles and even a few spare heatbreaks at your disposal (try to opt for the Teflon-lined kind unless you can afford the highly polished E3D heatbreaks, since cheap non-lined heatbreaks will jam with PLA).
Printed parts have never been a problem for me. The three printed components necessary for the P3Steel (two X ends and the X carriage) have barely any stress placed on them, and assuming the printer prints them well, with a decent number of perimeters and 50% or so infill, you’d be hard-pressed to break them. Also, I haven’t seen any molded parts that offer X belt tensioning braced by the X rods rather than the Z rods, which is almost critical if you want a proper, tight belt tension but want to avoid bending the Z rods.
There is the option of replacing the X ends and carriage with steel laser-cut parts, which eliminate plastic parts entirely from the frame and use them only for inconsequential things such as the endstop holders. I haven’t seen a need for that, though.
There’s no real need to overbuild it. It’s already a solid design and will already last a long time even after taking a beating. Just maintain it and treat it well, maybe do a few upgrades once in a while, and it will do everything you need it to for years to come.
Hope this helps!
-Karl
