Here's the deal, I don't think many people have theory crafted this out yet.
The Urbanmech WILL have significant quirks for its stock weapon system.
AC/20 Urbie with at least 50% cooldown quirk and maybe even velocity and range? The fastest engine you could probably fit in that is an XL80 (~50kph)
Here is an approximation on Smurfy:
ANANSI (AC/20)
(I used an AC/10 because the actuators are in the way. I added a medium laser and two AMS to make up for the missing 2 tons and 3 slots that an AC/20 would use. Using an XL100, you'd need to go down an extra 0.5 tons to be able to fit in the last heatsink, which is approximately an XL80)
AC/10 Urbie, again, probably with massive 50% or similar cooldown or other quirks that scream (TAKE ME, I'M VALUABLE!) you'd be forced to take roughly an XL145 (86kph)
ANANSI (AC/10)
LBX/10 Urbie, similar quirks, about XL170 (101kph)
ANANSI (LBX/10)
Those are all with only 2 tons of ammo and no backup weapons. In order to pack more ammo or bring a laser or two, you'd need to drop a few more engine ratings.
Add to that the fact that the Urbie will probably NOT be XL-friendly, and you'll be pressured into taking a STD engine (even slower), making the quirks all the more essential.
Sure, if you want to run a mini laser vomit Urbie that hits 125kph you can, but you're missing out on the quirks and you're probably running an XL (which again, IMO, won't, or at least realistically shouldn't be very safe an on Urbie)
PS. I really like that idea for 2x ammo per ton quirk, that'll really help. =]