There are a lot of variables there. Assuming the rig is well maintained and is in good shape, I'd say that it should work as well as a newer rig and the malfunction rate should be the same. I'm not addressing older main canopies at all here. Each one has it's own temperament.
There is a limit to this though. Really old rigs obviously are not as safe as the newer ones. Belly band pilot chutes, capewells and 5 cell reserves all worked... most of the time, but I'm grateful that technology has moved past these.
The newer rigs probably have some technologies built in that the older ones don't have that will significantly improve your chances. RSLs, MARD systems, modern AADs and some tweaks in the reserve pilot chutes have all pushed things in the right direction. YMMV. Of course none of those things will prevent malfunctions but if you do have one, your odds go up. The newer rigs are built for higher speeds and present less opportunity for a premature deployment.
I'd say that if you are looking for a rig, try for the newest rig you can afford but don't discount an older one without talking with a rigger about it. Pretty much anything (that is well maintained and in serviceable condition) within the last decade would be a good bet but again, you really need to be doing this with the help of a rigger. There is a lot of cheap gear that is cheap for a reason or just not right for you.
(This post was edited by ctrph8 on Jul 9, 2012, 12:15 AM)