I though some here may be interested in this happy ending story:
I finally have fully sorted M5 with 111,7K miles on the clock with $20k+ in all records ($11.7K me and $8.5k PO's).
I bought it in April last year with 109K miles on the clock. It had clean Carfax with exception of damage report (not accident) where PO managed to hook front bumper on some sort of obstacle in the parking lot and he used insurance for new OEM bumper.
It was always southern car with 3 previous owners and all records. It was in good shape cosmetic (somewhat excessive hood road rash by my standards) but it needed some mechanical addressing.
So, after I brought it home and slowly addressed all issues and added some nice upgrades: ($11K is parts only and countless hours of hard work as I've done everything by myself with exception of my neighbor TIG welding exhaust system).
After 14 months and 3k miles added, she has full bill of health:
In chronological order starting from the moment I brought it home:
Change all fluids with Liqui Moly products (oil, brake fluid, power steering fluid, diff fluid, tranny fluid). Notably, I left coolant alone as it just received new radiator with BMW coolant prior to mu purchase.
New Autocraft Gold battery (paid by PO).
Added VDO oil pressure gauge in place of fog light switch. Very OEM looking and quite useful upgrade.
New BMW OEM ZHP weighed shift knob and new coin storage box.
Euro style leather armrest with storage (seller from Europe, brand new leather!).
New OEM power steering hoses.
New Mahle OEM fuel filter.
New BMW spare key.
New all 4 Genuine BMW wheels, new spare tire kit plus new front Michelin Super Sport tires (rear ones just installed by PO).
New NGK 3199 spark plugs.
New BMW OEM rubber trunk mat and front mats.
New Bosch OEM MAF's.
New Bosch OEM O2 sensors, all 4.
New BMW OEM kidney grills.
New Spal cooling fan kit (fan clutch delete). This is somewhat custom kit I designed myself that I have on all of my BMW's and it works flawlessly. It uses extra sensor incorporated in the lower radiator hose and it triggers the fan if coolant returning to block exceeds 185F. it works well in tandem with factory 79C thermostat.
Full custom exhaust system (from front to rear):
Supersprint headers, high flow 200 Cell Metallic HJS cats, custom made X pipe, 2.5" Vibrant performace resonators, Supersprint Magnum mufflers. All components in between are SS T304 Vibrant Performance tubing and are TIG welded. It's worth mentioning that I've closely followed specs of full Supesprint exhaust system that sells in excess of $7K+.
According to Supersprint dyno, this is about 35HP upgrade which I haven't confirmed but butt dyno agrees and sound is awesome.
Also, I've saved about 80lbs as OEM exhaust while well built is a heavy monster plus OEM exhaust has 2 major flattened areas where flow is quite restrictive.
Completely new front and rear suspension:
Bilstein B6 struts and shocks (OEM springs retained) which pretty much keep the stock height with slightly firmer ride. All control arms front and rear replaced with Lemofrder ones. Tie rods and steering shaft replaced by PO. All mounts, spring pads etc are replaced as well. And of course, the alignment at end.
New LUK clutch and LUK flywheel with every single component replaced while in there: Rear main seal, slave cylinder, pivot fork, guide bushing, release arm spring, etc...all parts either genuine BMW or OEM.
New OEM diff input seal, driveshaft carrier bearing, driveshaft u joint etc...
New OEM BMW shifter kit replacing every single component.
New OEM BMW motor mounts.
New OEM BMW high pressure steering line.
New all 6 rubber brake lines (OEM FTE and ATE).
New genuine Sharp LCD for navigation screen.
New all 10 trim body trim pieces (4 x Genuine BMW, 6 x aftermarket).
New BMW driver side "M5" door sill (passenger side replaced by PO)
Just did oil change #2 at around 2k miles with Liqui Moly 10W60 and took advantage to replace Vanos screen. No noticeable consumption although I'm sure it probably would use some at extended oil change intervals.
$500 woorth of brand new spare parts not installed yet (found good deals): Genuine BMW water pump, OEM Pierburg fuel pump, genuine BMW instrument cluster plastic shroud.
I'm probably forgetting some smaller items.