The inline 5 turbo injected diesel engine is not your average old time truck engine. Maybe a gallon or so of #1 diesel (kerosine), but not gasoline. Even a small amount of gas in these Sprinters make the engine run incredibly rough. It's definitely not something you'd want to do if you have a busted motor mount, if you know what I mean.
Glow plugs are an easy and relatively cheap fix. They're about $20 each ($14 from Rock Auto, $15 from Europarts, $48 at a dealer). The glow plug module (under the battery, behind the headlight assembly) will probably have to be replaced, unless it's already been replaced with the new one before you bought it. The original module has a fuse in there that can be replaced by soldering a new one into it, but the new one is solid state. If you replace it with a new one, then you should not have to replace it again when another glow plug blows and takes out the module. You just replace the plugs that are bad and reset the computer code and you're good to go. The ones that haven't gone bad don't need replacing until they do, although with three already gone out, I'd probably just replace all five. The module is $165 ($250 at the dealer). It's something you can do yourself, or a mechanic can do it in half an hour. But, the glow plugs are prone to snapping off, so you'll want the engine as hot as it can get, and make sure the mechanic knows how to get them out if they break. I've replaced all five of mine, and they came out easily, though.
Next time you have the transmission fluid changed, have then clean the conductor plate and speed sensor, and the circuit board up inside the transmission. Drop the pan and there it is. That can get contaminated, cause faulty reads from the speed sensor, and can cause the transmission to go into a more severe LHM were you only get reverse and second gear. You can reset it by turning the key on and off, but you only get about 5 of those resets before it's permanent until cleared by the DRBIII tool.
Motor mounts are $25-$30 each. Replace both of them. Labor will be between $400 and $600 depending on where you have it done. You don't need a Sprinter guy to do that, as any competent mechanic can do it, as long as he can independently raise and support the vehicle while also raising the engine inside the engine compartment a couple of inches with an engine support. Seems kinda pricey, but it's dirt cheap compared to having to replace all the stuff that can be damaged by long term excess vibration.