It is well worth getting off of the septic system, if you can. Having the tank pumped out every two years runs about $3-400, and you do not have to worry about back-ups, should a tenant clog the drain field with grease - which does not sink to the bottom of the tank, but floats at the top to run off into the drain field.
When the drain field gets clogged, it needs to be jetted, which is additional expense. Jetting uses water pressure to propel the hose forward into the drain field, and at the same time clears out clogs.
While we are talking about plumbing problems: a product called the "Clog Hog" can be bought via mail order for about $300 to clear 3-4" sewer pipes of grease and obstructions, the next time you have a backed-up pipe. Clog Hogs are advertised in Popular Mechanics.
There are plumbers who specialize in installing sewer drains and should be able to give you a pretty good estimate from a site visit.
For interesting project photos and remodeling info, visit: http://www.BruceMoose.com
My advice to you is, if you like the house, and the septic drain field and tank passes inspection, buy the house and learn how to maintain the system.