I lived in a 1901 home for 12 years and replaced almost all of the junk trim with the nice Victorian stuff (that is a technical term). Here is what you need for your projects:
1. Liquid nails paneling and molding (LNP&M)
2. A brad gun for small trim
3. A Finnish gun for larger trim
4. Lite-weight Spackle (it has the consistency of whipped cream and dries hard like chalk) from HD ow Lowes - sold in the paint section (with liquid nails)
5. Caulking
First off, plaster walls will vary in thickness, a lot. All of your molding will need more holding power than your nails or brads will be able to provide (with or without the studs). Simply apply a a bead or two of the LNP&M and nail off your trim. The nails will provide the holding power while the LNP&M dries.
If you do not follow my instructions - I'll have an
'I told you so' waiting for you when your molding pulls away from the wall even though you spent 2 days locating all of the studs.