The Olympus OM system is a good one. The OM 1 and 1n can be converted to take modern batteries or you can use an adapter as others have suggested. Unless you are planning to use flash and want auto mode, the 1n and 2n are not really neccessary. The shoe 4 which is required on these 2 models is neither real easy to find nor inexpensive. Stick with the single digit models. The 3, 3T and 4T can get a little more expensive, particularly the 3T.
The lenses are generally outstanding. Some of the ones which were pricy when originally manufactured, still are. Olympus has a great macro system. As a collector and user of OM gear, ApUGers might want to contact me offlist if the have a burning desire for something OM. Bill Barber