You're on the right track. The subframe and PPI (pre purchase inspection) are 2 great starting points. Know that the E46 is susceptible to cooling issues, certain areas are prone to oil leaks (easily a DIY project), window regulators fail and some other knick knacks here and there. BMW's in general require more maintenance than the average toyota or honda. But if you are willing to DIY, which it seems that you are, then it seems like a decent deal. You may be able to haggle a bit and bring the price down a bit. But otherwise, it seems you are headed in the right direction.
One word of advice though, it is fun to mod the car. Believe me, I know. But make sure to be completely up to date with your maintenance before diving into the mod side of things. Otherwise most here will give you a hard time. Worry about functionality before aesthetics. Your car will thank you for it. These cars will last a very very long time if well maintained.
P46 Projectors + FX-R Retrofit + 6k D2S / 60 LED AE's / Smoked LED Tails / Smoked Turn and Side Markers / BSW 10" Sub /ZHP Knob / Matte Black Grills / DICE / Parrot + Connects2 / OEM Alternative Alarm Mod / Clutch Switch Delay Mod / M3 Rep Front Bumper / 3M Black CF Di-Noc Wrap / K&N Intake / 35% Tint All Around / Black Short Eurotray / 545i Shifter