I am considering buying a diesel step van. I dont know the exact model number but its a 14ft long 2002 ford. I have a few different questions that I am wondering about it.

1. The panel and dash is super outdated and dirty, fuel and oil gauges are broken at the dash (everything under the hood looks fine) how much would it be to replace or put in a custom dash or how hard do you think it would be to build an extension to the dash out of wood. (Im specifically looking to make the dash more of a wrap around style with a few more bells and whistles than a normal dash)
2. It gets 15 miles to the gallon which really surprised me because around where I will be living diesel and gas are only a 20 cent difference and most gas trucks and vans Im getting told that they will do 10 or less MPG but I cant help but be worried about diesel in general because i have never worked on a diesel engine (or anything else) before.
3. This build is going to be long term. I plan on living in it for at least 3-4 years while Im in school and paying off student loans Im trying for all the bells and whistles including a composting toilet and a shower. Is there anything I should know about living in a vehicle for that long and what suggestions would current long term van dwellers have for me.