My old man has a fitters ticket, and has converted a lot of cars. Getting all new tanks is not the way to go. Buy an older car (look for XF's, they're cheap and alot of them are on gas), already converted to gas, with an in-date tank.
There is only a great power loss with LPG when it is improperly tuned, or run on a dual-fuel combination (like taxis). Otherwise you can get the same performance out of a LPG engine. They really are not that complicated, the only difference is the convertors and electrics...
Oh yeah, by the way, I think Mythbusters actually shot a gas tank, and it didn't pierce. Because it is a completely sealed tank, no matter what, even when you fill it up...