For FD's, going single will most likely be more reliable than the orginal twin turbo system. The stock turbos have a lot of problems, which is why many FD owners switch to single. Some of the single turbos (like the GT series) spool up quite quick, almost as quick as the stock ones, so if you choose the right one, you won't have to worry about lag so much either.
That being said, the most important things in making a car reliable will be reliability mods. A bigger radiator will help a great deal, since the stock intercooler isn't really enough for even stock conditions. You'll need an intercooler as well, since the stock one just won't cut it. A bigger single turbo will need more fuel, so you'll need appropriate fuel mods. A boost controller will allow you to control your power, and you'll need a good boost gauge to keep an eye on things, so you don't spike and blow your engine. If you're in a hurry, a turbo timer might be a good idea as well. And probably the biggest thing that will lead to reliability will be good tuning. A single turbo engine will require a standalone ECU also, so you should figure that into the price as well.
As for the price,
Single Turbo Kit (GT35R)- $3000
Intercooler- $1000
Radiator- $400
Fuel System (Inectors/Rails)- $800
Fuel Pump- $300
Standalone ECU- $1000
Boost Controller- $400
Boost Gauge- $60
Turbo Timer- $100
That works out to about $7000.00, and that's not including little things, like exhaust, vacuum hoses, etc. It also doesn't include money for tuning, which can also set you back a fair deal. So it's not a cheap venture at all, but so long as the tuning is right, and you take car of the car, a single turbo FD can be just as reliable as a stock one, if not greatly more reliable.
