Welcome!
In regards to getting a 2500 or a 3500 dually: The dually will carry more payload, however, I have heard folks complain about a stiff ride when these vans are not loaded enough. Several mainstream van conversion companies have started using the 3500's because their conversions are quite heavy. SMB conversions are not light, however, I've yet to hear of one that overloaded a 2500 van. I plan on going for the 2500 Sprinter when I get one; the 3500 looks like overkill to me. The factory suspension upgrades seem like a good idea , although John at the Sprinter Store suggested his aftermarket suspension kit to me. Might want to contact him as he is very helpful about all things Sprinter:
http://www.upscaleauto.com/sprinter_store.htm
Prices on the 2008 van are rather variable. I suggest doing a nationwide internet search. AFAIK, Victorville Motors in SoCal has a number of 2008 vans in stock. SMB West also has five 2008 2500's that are available for conversion. You can find more discussion of Sprinter pricing and an amazing amount of info on Sprinters at:
http://www.sprinter-source.com
Don't worry about not being mechanically inclined. This forum and the Sprinter-Source are a wealth of knowledge. I've learned a TON just by reading the forums and taking notes! The Sprinter is a more complex vehicle than the Westy and it also is a diesel which leads me to some finer points on diesel ownership:
Fuel-Keep your Sprinter happy by buying fresh,clean diesel fuel preferably at a truck stop. Diesel fuel does not store well and so truck stops are your best bet b/c the diesel gets sold quickly. Of course, there may not be a truck stop always around, so it's prudent to pick a gas station that has fairly high traffic. Be wary of a station out in the middle of nowhere that sells diesel. Don't fill up with diesel when the fuel truck is filling the stations tanks with diesel fuel. When the station diesel tanks get filled, it stirs up water and other crud that you don't want in your rigs tank.
Fuel filter-This must be changed at the proper interval (see owner's manual) It is not a difficult DIY job and there are some great instructions on this fine Sprinter blog:
http://www.ourexcellentadventures.com
A dealer can do this or find a qualified independent mechanic. I tend to steer clear of dealer's so I would be inclined to do most jobs like this myself unless I have a good mechanic nearby.
Oil Change-Should be done at the regular interval which is said to be 10,000k miles. Low ash oil must be used to keep the emissions system happy. This info is in the owners manual.
Turbo hose-Sprinter specific 2007+ model years- The turbo hoses are prone to cracking which makes the van slow way down and go into limp home mode. The fix for this is easy and permanent (scroll down the pagea bit):
http://www.riordanco.com/our_products_catalog.htm
Don't be discouraged by all the Sprinter issues that you may encounter on the forums. Remember, people rarely post when their Sprinter is humming right along! If you carefully maintain your Sprinter, you'll really help ensure that your rig runs well.
Since you are going to be buying a Dodge Sprinter, be sure to check out the Sprinter-Source regarding warranty issues. There seems to be confusion as to whether Mercedes will honor the extended Dodge Sprinter Warranty. As you may know, Chrysler and Mercedes got a rather nasty divorce and as of January 1st 2010, Mercedes is selling the Sprinter through a select number of Mercedes and Freightliner dealers.
Your idea is not a bad one. I recommend doing a TON of research, talk to SMB, check out the forums mentioned above, take notes and have fun! It has taken me about six months of forum reading to really get into the nuts and bolts of the Sprinter.
HTH
Rick