There is LOTS of reading over on this forum for you.
http://sprinter-source.com/forum/
25mpg is not going to happen in a T1N sprinter (02-06) unless you drive under 65mph, and have a smaller van. In my 2004 140" high roof sprinter, my highway mileage at 70-75 is 20.5mpg repeatably.
The newest I4 sprinters (2014+) can get that kind of mileage, but they are not cheap.
02-06 are T1N models, they all use the very reliable I5 engine (OM612 and OM647 designation). 07+ are NCV3 models, they are nicer inside, ride better, and have a few nicer options. NCV3 models use the v6 engine, and have more emissions equipment to deal with. T1N models only have EGR, no DEF/DPF to deal with.
T1N sprinters are fairly reliable, but they can be sensitive to abuse, and poor maintenance practices. They also have ******%TY paint if they are white. The other colors (especially those with metallic paints) are better.
Most mechanics (and many dealers) have no idea how to diagnose them. Not that it is very hard, and the T1N drivetrain is easy to work on. A $200 diagnostic scanner (which can be used on many other vehicles) combined with reading/using the sprinter-source forums will provide you with more expertise than most shops.
There are minor differences between years, but the T1N sprinters are all very similar. The biggest difference is between 02-03 and 04-06. The engine changed from OM612 to OM647. The core engine remains the same, but the engine management and accessories changed. Turbo becomes electronic instead of vacuum controlled. EGR becomes more reliable, intake manifold changes, and a wideband O2 sensor is added.
