the rust is what gets them, i would get under it. it would be great if you could take it to someone with a lift. these cars last 2-300k miles easy, perfect example a year or two ago my inlaws had a 96 se that went to the scrap pile. it had maybe 250-275k miles and ran perfect but the frame was starting to bend and tear. so they found a newer lesabre(almost the same car as a se bonnie) that had very little rust under it, i crawled around under it. the owner said it was a southern pa car. they just use sand.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock