If you can fit them, the larger 2 5/8 are easier to read. Mechanical gauges are best (take a little longer to install), but the electrics will work fine.
If you're only mounting a fuel pressure gauge under the bonnet, you don't use one of these large expensive dash mounted gauges. There are small 1 1/2" specific fuel press gauges for mounting near the carby.
This is the fuel gauge you need..........or one like it. Are you using an electric pump? (ie: want to set reg pressure or just see if your mechanical pump is keeping up at high revs?............if so, you won't see it when you need it).
These liquid filled Jeg ones are good. US$20.
Just another point...........the original fuel press gauge you mentioned comes with an isolator. This allows you to mount the gauge inside your car without fuel going to the gauge. If you're only mounting it under the bonnet, you won't need this part.
Look into mounting a gauge holder panel where the centre dash speaker sits on the XD/E. I'll find a pic for you later.
Voltmeter gauges are quite handy. It's saved my bacon several times. Hows the ALT system operating? You can see and anticipate electrical problems and monitor system load.
Style: what ever excites you.
The 5" tachos mount very securely on the drivers side pillar...........make sure screws go into steel, not just plastic. Screwing them to the dash allows them to vibrate heaps.
WRT the vacuum gauge, you can get handheld ones that plug into the carby base plate when tuning...........then remove when finished.