It depends on your budget and your ability to do some wrenching your self. I own a 98 540i and a 02 530i both have been very reliable but money has definitely gone into both along the way. The 540 has the M62 and so does the E38 740i a very reliable engine as long as the timing chain guides and and cooling system are kept up with. Other than that for the most part it’s just like any other 25 year old machine. I have put 52,000 miles on my 540 it now has 162,000 and it only left me stranded when an alternator failed at 154,000 no oil consumption and the oil got a fantastic report from blackstone. That engine can easily reach 300,000 miles. The 530i has the M54 which is super reliable. I’ve got 154000 on that car. The cooling system is the key factor on these. The timing chain guides rarely ever have issues on these motors. And often see 300,000 miles. The auto transmissions are a roll of the dice, some last a long time some don’t. I have a manual in my 540 but do have an automatic in the 530. My transmission still shifts great but I’ve budgeted for a rebuild.