I'd suggest going to a big-box RV dealer, preferably non-Camping World. Go to their parts & supplies store. Ask for the big catalog that they have. It may be free or $5-$10. It's actually a mail-order warehouse's catalog with that dealer's imprint on the cover. If you can find the items online, you may save 10-20-30% over the catalog prices.
Start thumbing thru it. You'll need something from most categories, usually the smallest, to build a fully functional B with bathroom and kitchen. But with that catalog, a hightop van, and a Home Depot or Lowes, you can build a camper limited only by your tools, skills, patience, and credit line. Think thru each system. Example: for sink & shower, you'll need water & waste tanks, pump with wiring & switch, plumbing, water heater, sink. Water heater usually means propane, so add an LP tank and plumbing.
I think you'd asked elsewhere about parking downtown. It's easier and you have more choices in a 17' (standard length) van, than a 19' extended van. That's a basic decision you need to make right up front. RT makes a very nice smaller but full B van in the 170 on a 2500 chassis. You can get by with a 250/2500 in that length. For a 19' B, you'll almost surely want a 350/3500 by the time you
load it up.
It's a big undertaking with a lot of details, but not un-doable.
Jim, "Mo' coffee!"