Not dumb questions...
The 1, 2 and 3 on your oil pressure gauge represent the European measurement of pressure (Barometric pressure or BAR for short). 1 BAR = 14.7 psi (or 15 psi rounded). So, 1.5 BAR (at idle) represents roughly 22.5psi and pegged (guages reads 3 at anything over 800-900 rpm) should mean around 45 psi.
As far as the o-ring on the dipstick, I'd say you should be able to find something adequate at most parts stores. However, if you've got a dealership or a good independent mechanic around, you can pick one up there. I would guess under $1 (of course, something like that shouldn't cost more than 5 to 7 cents...)
Don't know nothin' about your oil filter though... (sorry).
Scotts Valley, CA.
1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles
2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)