Unless you got a super-duper deal from your friend (which would probably not make him your friend anymore) I would pass

2000 was the 1st manufacture yr for the x5--so you're getting a model with a high likelihood of electronic gremlins and such.

Do a search for common x5 problems and if they have not happened yet, budget for them

Ask your buddy for service records and pull the OBDII codes with a code reader to see if there are any problems.

Re: your known issues
1) driver window reg sticks down if fully lowered (best place to buy part and approx cost of install from indie)? guess $300 parts plus labor - You're in Hawaii right? so your choices are limited
2) top windshield seal deteriorated, cracked - never heard of this before. Sounds expensive for the labor
3) left turn signals not working (unbelievably bad engineering to access the bulbs from what I can see)?? any advice welcomed - common problem, cheap fix is bulb plus cable DIY like 40 bucks
4) service engine soon light on (assume fluid changes but could be more even though it runs fine)
This means you will need a Service Inspection II (normally at 60k) - this is the bigger more expensive maintenance item - figure 600-1k
5) dead pixel problem instrument cluster, about 35% bad pixels (big expense?) - no idea
6) hazing of headlight plastic much worse on left side. - can fix with sandpaper DIY but it takes time