Hey, Welcome to the forum.
e46 M3's go anywhere between 150k to 300k.
The former being an old abused M3 and the latter being a young well looked after M3.
I promise you now, if you go for the cheaper M3, you're only going to pay in the long run with heavy maintenance bills as these cars are not cheap to maintain if everything starts breaking together at the same time. The expression, cheaper is more expensive applies here. Things that you will want to look out for, are the rear subframe tears that appear ontop of the boot floor and where the rear suspension mounts. These are signs of a "Well used" e46 M3 and I would steer well clear of this. Another sign is a noisy vanos unit. Listen for this noise when starting up the car --
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This is caused by exhaust cam hubs rattling on the vanos unit and can be catastrophic if they break or fail. Stay away from this.
Remember, cars before 2003 where prone to having big end bearing failures on the crank. If you do decide to buy an M3 from 2003 or before, check the service records with BMW to ensure the car was indeed recalled to have these bearings replaced.
If you are up here in Jhb, A1exander from Axis Engine works is the gentleman to see regarding keeping your e46 M3 and it's S54B32 engine in good healthy nick. He can fix the above Vanos rattle. I would suggest the first thing you do when you buy your e46 M3 is come and visit A1exander and have the vanos seals, anti rattle and exhaust hub job done. This will ensure you will never have vanos issues. The kits available today improve the design flaws of the S54 double Vanos.
I can speak from experience as I had a full engine rebuild with them and many other users on this forum who own e46 m3's have had their cars revitalized and given the all clear by A1exander. Billybob's M3 was down on power and having rough idle. A1exander replaced Vanos seals and found intake leaks on the intake manifold. He fixed this issue and the car gained 10nm torque to the wheels and a smooth idle.
CSL's are a special breed and obviously come with a hefty price tag as they are collectibles. Expect to pay R500k and insurance to boot.
If you are wanting to modify and do upgrades, there are many forum sponsors who supply tuning and other mods, take a look at the advertisers section. I personally use Project Shift to import all my engine mods, like forged pistons, rods, racing valves, my supercharger kit etc.
Using Billybob's car once again as an example, Project Shift installed a Custom CSL intake on his car with Alpha N tune, he gained power. You will need to speak with him in regards to how much exactly. Project Shift have also installed Alpha N software on a supercharged e46 m3 that before coming to them, was undrivable due to tuning. They are currently building a turbo charged e46 M3 at the moment. Speak to Burgy for more info.
Hope this helps you in your decision to get an e46 m3.