I have a slightly more costly (but still el'cheapo) aluminum mountain bike from WallyWorld so mebbe I can help both of you a bit lol
Overall - "el'cheapo" bikes from Walmart are fine, IF you check the welds on the frame and pick one with good, heavy beads. The frames themselves are usually good, it's the various hardware, controls, shifters, rims, etc. that tend to be garbage. This isn't so much a problem as you'll prolly be replacing/not using it much anyway (like shifters lol). The rims/wheelset are NOT to be trusted, but everything else is prolly ok.
Tubes - DON'T even ride the thing with the factory tubes in it if it's an el'cheapo - they're sure to pop. Both of mine split right next to a seam within 2hrs of riding, one right after the other lol. Good tubes are cheap enough.
Tires - knobbies are fine, they slow you down jus a little but if you're careful to pick out a set with interlocking center tread the ride is smooth and wear is MUCH less. (see pic)
Bearings - other than replacing the entire wheelset with higher quality, I'd say just slap some good
marine grade grease in there. I've over 600mi on my cheap bearings and they are doing fine, but the rims/spokes themselves worry me... a LOT actually.
Aluminum frame - I'm quite fond of my aluminum MB, but I'm not sure I'd recommend it to anyone. Definitely do NOT do the "drill-through" method of mounting the engine as this can lead to frame failure even with a steel bike, an aluminum one would only be more likely to fracture. This is a problem as most aluminum bikes have an oversize/oddly shaped Dtube. Giants are real nice bikes - but most I've seen have a HUGE Dtube...
Be very VERY careful if you decide to use aluminum. They're great, but things like automotive exhaust clamps (sometimes used for mounting motor) will surely crush the frame and ruin its structural integrity.
Another note about aluminum bikes, be wary of wear and chafe. Your chain occasionally slapping a steel bike will ruin a nice paint job it's true - but on an aluminum bike it'll cut through in short order. Every wear point suffers from this, I've kept on top of mine (I hope) - but be warned