Welcome to the forum.
For a first time D2 owner, you may want to consider something a little more road ready. I understand the appeal of fixing one up, but there's quite a bit that can be wrong on these trucks after they've been sitting for long periods and not being familiar with the pitfalls and traps can be very frustrating.
1. If you're bent on that truck though, there are 2 issues you need to be concerned with. The easiest to validate is the frame. Check the rear frame for rot/rust. The truck looks like a CA truck and you're probably good, but then that truck is 20-years old and if it's more than a 1-person owner, it could have done a tour in the mid-west or northeast.
2. The second and you already seem to know, the Engine - i.e. it's a 2003. At the same time, it has over 100K on it and the likelihood there's a bad motor in there is slim at this point. If they tell you anything about it having overheated, be VERY leery! As you seem to be aware, the GLARING item on this model year is the Engine. I would make sure that engine runs, and runs well. Don't just start the truck up and let it run for 5-min and shut it off if it doesn't overheat. I have personally had one of our trucks sit at idle with a cracked block and take 15-min for it to finally overheat (it was more than a decade ago and the thread is likely still on this board...heh). Bring it up to temperature and listen to every sound.
If all that's good, honestly, everything looks very straight on that truck and worth the ask. It's extremely clean on the inside, which will limit some of the nickel and diming, though who knows what else isn't functioning properly and how close you want the truck to be to a daily or reconditioned status? Our trucks are well known for door latches breaking. Window regulators bending very easily. Sunroofs leaking. Sunroof motors locking up, etc. Not trying to scare you, just set your expectations realistically and let you know the items the seller listed are just the tip of the iceberg - like any 20-year old truck sitting in a state of disrepair. You are likely going to be replacing anything and everything on that truck that is made of plastic (reservoirs & coolant connections), rubber (brake lines, vacuum lines, hoses, etc.) or insulated. But it looks like there's good guts on this truck.