You got a very good deal
@Naplescaptain! There are no used '18 Denali XL's with <40K miles out there for mid $50K range, unless they're lacking a few options (no power retractable steps, 20" wheels and not 22" are the major ones) and they're only 2WD.
Does your Denali XL have 4WD?
Power retractable side steps?
Does it have factory 22" wheels or 20"?
Even if yours is 2WD with standard side steps and 20" wheels you got a great deal given the model year and mileage.
Just as
@swathdiver stated follow the severe maintenance schedule. Especially on things like transmission, transfer case, and diffs.
For your 2018 GMC Yukon XL Denali following the severe maintenance schedule at 45K miles it's time for
- Replace interior cabin air filter
$25 filter and about 15 mins in your driveway with simple hand tools. Plenty of DIY how to youtube videos.
-Replace engine air filter
-$40-$90 filter and again about 15 mins in your driveway with simple hand tools. Plenty of DIY how to youtube videos.
- Transmission Fluid flush and change.
- 4WD Transfer case fluid flush and change.
If your '18 XL Denali does have the power retractable side steps be sure to keep the mounting points and hinges lubricated. I use a clear silicone spray lubricant that seems to work well.
I wouldn't sweat the lifter thing too much. Plenty of members on here have had to have them replaced and if the work was done at the dealer then all OEM parts were used and it was put back to OEM specifications. I had a lifter go bad in my 2015 Yukon XL Denali at about 78K miles, it was all covered under warranty, and I'm now at 130K and haven't missed a beat.
It is worth noting that I have had a Range AFM disabling module installed on my XL Denali since about 40K miles and I still had a lifter fail at 78K miles. So disabling the AFM electronically via a tune or "Range" type module is not a solution; Does it perhaps prolong the life of the lifters by preventing them from activating AFM "4-cylinder" mode? Sure probably, but it is not a solution or fix.
Did the dealer quote you $1,800 for a New drivers seat bottom cushion cover, a new set of rear shocks and new front stabilizer link ends parts and labor? If so, that is surprisingly a damn good deal. Most dealers are quoting more than that just to replace the rear MRC shocks.
As for replacing your shocks; You have the Z95 suspension package which is the Magnetic Ride Control (MRC). I would recommend OEM parts ACDELCO Part #84977478. These can be found on Rockauto for about $450/ea (they used to be about $360-$380/ea.). In todays raw material environment though you will find that it is common they occasionally will be out of stock. Just keep hunting for them. Be aware of counterfeit aftermarket knock-offs on Amazon.
The dealer will try to charge you $700-$950/ea. for these shocks plus labor. You are better off checking with a local Tire Discounters or the like and just asking them: "I need my rear shocks replaced, if I supply you the parts will you perform the labor only?" I have a few Tire Discounters and auto places near me that will do this. You'll save significant $ over dealer OEM part mark-up and dealer labor.
Why does it need a new drivers side seat bottom cushion cover? Is it ripped? Torn?
Plenty of great information, advise, and direction on this forum for the full size GM SUV Platform.
Hope this helps. Congrats on the purchase!