Well, from what I understand you're going to have to upgrade the turbo itself at least moderately, if not to a full Big Turbo (BT) setup. A GT28RS may get you there with appropriate supporting mods. One good thing to check out is the post where everyone was listing their car and its mods (I don't know for sure, but some people probably have listed their measured output as well). For parts, you can rely on
www.purems.com, www.ecstuning.com, and other forum sponsors (in addition to supporting the companies that support the forum, they're also well-known and have good prices and excellent reputations). Also, check out
www.034motorsports.com - they have some wild projects that they've done (including big-turbo VR6 transplants that can net you well over 600hp at the wheels), so they definitely know a thing or two about building the B5 cars. They may have the info on the site, but if not, you could probably call any of those companies and get input on what it takes to reach that goal.
In the most basic sense though,you're going to need a larger turbo, a front-mount (or high-quality upgraded sidemount) intercooler, injectors, ECU reflash (get the one appropriate for your turbo - check with APR about their specific tunes, and you can use that as a starting point for selecting the turbo, since it's much easier than spec'cing your own turbo and then having to have a full custom tune done, even though that may well end up netting more power), probably a manual boost controller, intake, upgraded diverter valve (DV), test pipe, downpipe, exhaust system (3" all the way through is best), and you're going to want to get a stronger clutch assembly as well - I highly doubt the OEM clutch, made for the 150-170hp output of the stock motor, will handle 300hp). With that much extra go, invest in an A8 brake upgrade kit and better pads as well, to rein in the car when you need to.
There are other miscellaneous areas to address as well, and replace/upgrade parts as needed. I'd get a new aftermarket snub mount for sure, to control fore-aft rock of the engine, inspect the motor mounts and tranny mounts, and replace as needed, and you're going to want a quality boost gauge to monitor the goings-on in your car. If I were you, I'd get an oil pressure gauge as well, since our cars have an oil temp gauge, but only a low-pressure warning light. There are a variety of mounts for these cars, including dash vent pods, column pods, and A-pillar pods, so you can choose what you'd want.
And last, I have to say this for us 2.8 guys "just because", you could just sell the 1.8T and get a 2.8 30v with a PES supercharger and you'll have your 300hp:)