As far as I know they ARE street-legal - wheeled military vehicles still need to meet DOT and EPA specs. Humvees have safety glass, lighting that can be seen from all angles, seatbelts, and since they have a GVWR of about 8000lbs they're in the same emissions class as a 3/4 ton truck. Some people will say that they don't meed side-impact specs, but I say "[b]prove it![/b]" - a Jeep Wrangler isn't any stronger in a side impact and it's street legal. The only things I could foresee a humvee needing to be 100% in compliance are to have license plate lights, backup lights, and a 3rd brake light added, but only if the humvee is from a model year where light trucks were required to have the 3rd brake light.
Now, what would I do to it first? [b][i]Install a block heater and use it whenever the temp drops to 30F or below![/b][/i] Put a better fuel filter/water separator on, like a Racor 445R, and ensure it has the fuel heater option. If it's a 4-seater I'd run an auxiliary electric heater in the back, especially if you have the cargo area cover. Convert the military glow plug system over to the 85-93 electronic Chevy/GMC glow plug controller (not sure how it'll work with 24V), or just use a manual pushbutton switch that energizes a relay which in turn sends power to the banks of glow plugs. Use AC13G glow plugs are 12V plugs and the tips won't mushroom and break off when you're changing them. Trust me, you do NOT want to have to pull the heads on a humvee 6.2L in order to get broken glow plug pieces out of the cylinder (sometimes you can pull the injector out and fish the pieces out through the hole.) I could go into a lot more detail, but these are the very first things I'd do.