I reckon straight LPG is awesome.
Gas can vary in quality from place to place, but generally is very good. My personal favourite is kleenheat but it hard to find servo's stocking it. And yes, if you get a bad tankfull you'll know about it!!!
LPG needs hardened valve seats but so does unleaded, so you going to have to do it either way in the next few years, Stainless valves are also reccomended, but a high performace ULP engine is best to have them too.
Top of the line gas research systems are extremely expensive.... around $4000 tank to throttle body, installed. There are much cheaper options, but if your going for all out power then the GRA kit is the best way to go. Also second hand systems are generally very cheap.
Other than the above there is no other significant costs to an LPG engine over say a hot PULP engine. Both require hardened valve seats, regraphed dizzy, single piece valves, big cam and associated valve gear and both run higher compression than stock.
LPG can handle very high compression ratios with the right cam, but for most applications 10.5:1 is enough. I would not be expecting wild power gains over petrol, but built right they can match petrol engines with the added advantage of costing $20 to fill each time!
Although the anti-LPG crowd will tell you its not worth it, my engine is reliable as it gets, costs bugger all to fill up, makes plenty of power and as soon as I get the cam sorted and upgrade to a dual research setup i'm sure it'll be running side by side with them.... hopefully better