
Sunny,
An E39 M5 is an amazing car but do not cut corners on purchasing one. With simple sensors and mafs that go bad you can be down as much as 30-40hp and not even know it. Alot of them are still fast but not running to their full potential because they have not been serviced properly. A proper M5 can easily run low 13's and trap upwards of 108 MPH. Also, don't believe EPA figures as most guys seem to be getting from 24-26mpg on the highway around 75-85mph.
Have money if you want to make it run. Supersprint headers and full exhaust can cost anywhere from 2k-4k used (for the full system) but can get you as much as 40hp. Intake, pullies, throttle bodies and proper software can get you around 390-420whp. If you take care of it, they will be very very good to you. I have a close friend with a M5 track car (now retired with over 135,00 on it and probably close to 30k in track mileage since 2002) that still runs like it's new. With stoptech brakes, Dinan 3.45 LSD, intake/exhaust and SSR comps on RA1's he was turning 1:40-1:41 laps easily on Road Atlanta and 150+ on the back straight. The car is quite fast for what it is.
E55's and S55 are much faster cars in straight line. With bolt on's you will really have a sleeper. Check on MB boards as with Kleeman kits those things are running 11's with tires. You really need to drive Benz's though. They just simply arn't very rewarding outside of being beautiful luxury cars and wicked straight line cars. In terms of steering feel, pedal feel (the dreaded torque converter etc.) they are dead feeling. I also have a buddy who use to track his S55 Kleeman on Road Atlanta but after 2 or 3 laps it would heat soak terribly and he would have to back off....It was not a track car,nor was it very exciting to ride in but it was cool to see it run 155 on the back straights and hang on with Vette's for a lap.
The jags are great but even more "soft" then the benz's if you ask me. There is nothing really special about them to me other than their beauty and the nice blower on the 4.2 that can be turned up some. You can mod them but its expensive and they just really are not very exciting cars at all. If your going for class and something different, this would be a good choice.
I really am not fond of the LS400's. The motors are fantastic but the only thing there good for is cruising around and having the badass VIP potential. I'd highly recommend a newer 02-03 LS430 sport as their prices are way down and they are MUCH quicker and so much nicer then the 400's.
740i spors (00-01) are one of the best BMW's made if you ask me. There is no question that i'd take one over an LS400. They actually handle very nice for there size. I've actually autocrossed one for an entire day and had no trouble throwing it around and keeping up with less experienced drivers in much smaller cars. Also, it has that 4.4L V8 which is mod friendly and one of the best sounding motors on the planet. The later "sports" came with 3.15 open diffs as oppose to the older 2.81's. They are significatly faster (14.4-14.6's vs 14.7-15.0's) in the quarter mile. I've seen peolpe with simple exhaust (you'll drop 100lbs, chip, intake mods, removal of spare) run high 13's in those tanks