We have that same floor plan...rear twins and love it for now with kids...probably even more when we won't have to fold out our bed every day.
we paid $7000 for ours three years ago...but knew some things had to be worked on (mostly be self)...probably about $15,000 cash invested plus numerous hours of "love"
Things you might ask yourself:
Can you do mechanical work and get parts locally for a 1978 GM P-30 chassis? It is afterall a 25 year old truck that you will have with a very nice (I hope) camper attached.
Do you have time to tinker and tweak the chassis, interior, exterior, systems?
Is this for full timing, tailgating, a two-week vacation per year or numerous 2-3 day weekend camping trips?
Things to consider...Definitely miles on the engine, tire age - not whether they have tread, air suspension operating, generator able to fully power roof AC unit and run for more than one hour in GA heat, Appliances & systems functioning (AC, Fridge, stove, furnace, toilet, water lines, water pump, water heater), windshield leaks, gray & black tank leaks, floor rot, cushion condition, upolstery condition, flooring condition, P-30 chassis condition (axles, brakes, shocks, steering, exhaust), and DASH AC.
Many things are cosmetic or for aesthetics
...others for comfort
... and others down right neccessary for comfy road living.
Best of luck!!! We (I) have loved ours every day since we bought it...had to do some convincing with my wife...but she's come around.