Few things I learned with 98 Ram long bed with an ARA top that has the back door:
1. Paint is hard to match on a new one, for the older trucks I would look for used.
2. They are available in various materials and grades and shapes and heights. The working ones for carpenters, etc., are usually less expensive.
3. Various styles of windows, doors, side openings are available which will affect price.
4. If you have a new one, be sure to include in contract that paint will match. If done at factory, gas fill door, can be sent in for match.
5. Various hangers are available that can be mounted to roof to carry clothes, wood, fishing poles. Again, go over options and locations and direction they will face, front & back or to sides.
6. Depending upon your usage, you might want to see what's available in lighting for interior.
7. If you need 12v in camper, check on having dealer run a fused 10 gauge power line for you if not able to do it yourself. Jacks for cigarette lighter plugs are the most common for spotlights, coolers, compressors,etc. Factory wiring is not heavy enough and you can also screw up the truck's computers.
8. If have any thought at all so far as sleeping in it, make sure the windows will accomodate screens.
9. For secure storage, you can mount a steel or diamondplate box to the floor and put hardened padlocks on it. Then can unbolt and mount it in the next truck rather than having model specific storage that won't fit the new one.
10. If you put an alarm on the truck, consider a separate one for the back end, with an ancillary battery and siren in addition to regular power.
11. For any of you with bad backs, you might want to look at the combination door which has a small plexiglass door that opens like a regular door. Additionally the whole thing is hinged at the top and pivots up for access to the entire opening. The tailgate gets stored in the garage. Lot easier on the back than reaching over the tailgate or lifting that extra height if you don't lower the tailgate.
12. If you're at all handy, you can make your own cargo holders out of 2 x 4s and save some money on the commercial ones, or fancy bed liners with recesses for the wood.
13. Check out the installation for water tightness,tailgate too, not all fit perfectly and it may take some gasketing or silicone to finish the job.