Hi Oliver,
The Stanley 45 is primarily used for grooving and dadoing, beading, and cutting t&g. It is not meant for doing more ornate mouldings. The Stanley 55 is designed to do more ornate stuff - look at a picture of the cutters offered to get an idea of what it can do. Both planes are quite cantankerous and fussy to set up. They work marginally well, as there is no bearing surface ahead of the cutter, which leads to tear-out and ragged moldings. See these links for more info about the planes:
http://www.supertool.com/StanleyBG/stan6.htm http://www.supertool.com/StanleyBG/stan8.htm Depending on what you want to achieve, you may be better off in the long run assembling a set of wooden moulding planes. You can do a lot with a set of hollows and rounds, combined with a plow plane. Wooden beading planes and dado planes are quite common and fairly inexpensive (and fun to use!). Simple moulding planes (like ogees) are also fairly common and inexpensive. Complex moulders are more expensive, but are available, especially if you are not trying to match an historical piece.
If you do opt to go for the 45 or 55 (which I can understand because you do get a lot of options in one fell swoop), other brands, like Sargent, Wards, Craftsman, etc. can often be had cheaper than Stanleys. They work equally well, and some were even made by Stanley. I've got enough bits here to put together two working 45's. If interested I could send some prices.
Cheers,
Dan
