Hi Tim.
We own an Apollo and a Navara, have done now for 7 years so plenty of experience. There are many things to consider.
First off, let me say that we love our Apollo/Navara combination but there are pitfalls.
Clamping the camper to the truck bed - I can only recommend strapping or clamping down to the chassis so if you use eye bolts they need to be fixed through the truck bed to the chassis. The reason is that the tub is only fixed down by 4 bolts and is designed for vertical loads whereas the rocking motion of the camper will subject the tub to vertical loads due to the seesaw action.
An important point to mention with the Apollo is that the strap down points (it was designed for straps) are outboard of the tub and therefore the routing of the straps is problematic as you need to take them over the top of the tub if your tie down points are somewhere inside the truck bed, at least that has been our experience with our particular Apollo and Kingcab combo, this didn't really allow a secure strap down because of the strain on the sides of the tub and the fact that ratchet on the straps tended to fowl the top of the tub. After having the camper shift in the bed a couple of times we looked to an American solution.
Check out the Torklift website for tie-downs and the Fastgun turnbuckles, this is what we have, pricey but rock solid. see
http://www.torklift.com. This system clamps the camper down to the chassis and no drilling required as the brackets use existing chassis holes. Yes they do make them for the Navara, it's called the Frontier in the states. You will need a tow hitch fitted as the rear tie downs attach to that, whereas the front ones are directly bolted to the chassis.I can let you have model numbers as a guide but ✖ check with Torklift as it's been 6 years since we bought ours. The Torklift tiedown chooser page is here
http://www.torklift.com/fitguide5.php
You can see a picture of our Tiedowns and Fastguns here.
The Apollos that were designed to fit the long bed King Cab are I believe longer to take advantage of the extra bed length of that truck, the Double cab model bed is a foot shorter, so is the old truck a double cab also? If not you will probably have too much weight over the rear axle as you will end up with excessive overhang at the back.
The Navara springs are a bit of a compromise between load carrying and comfort, we found that with the camper on and loaded we had a only 1 inch of clearance between the bump stops and the axle so we were constantly riding on the rubber stops at the slightest bump. Fitting air suspension vastly improved the comfort and handling and ride height, an extra leaf spring can be fitted as an alternative.
Leg problems - It's possible but unlikely that both rear motors have packed up, you can easily connect the motors to a 12 volt battery to test them or swap the motors from front to back to test them, earlier designs used three counter sunk screw to hold the motors on, later designs are removable without the use of tools and yes you can take them off whilst the leg is in use, remove the screws and lift out the assembly, it just plugs into the top of the leg using a keyed adapter to locate it. I will have some spare motors in the near future as I have bought 5 new ones from China, but have yet to fit them, when I do I'll have 5 spares. The older style motors are BMW folding hard top motors, at least the ones on our legs are.
Check out the wiring to the non working motors, if they are like ours were originally the connectors were not very good and the leads were also connected using crimps which can corrode and part company over the years.
Roller blind - The windows Apollo use are generally industry standard parts such as Seitz and Fiamma. Our windows are Seitz with integrated blind and flyscreen. Window spares are readily available on line - they are not cheap. The worst case scenario would be a complete new window. If this is the case it might be worthwhile checking out the caravan breakers on line for a second hand window, just take the window make and measurements and search online on the manufacturers website for the correct model so that you can track down the correct replacement.
Seek advice from Apollo by all means but I can't and won't recommend that you let them to do any work. We left our camper with them last October to do some repairs and they didn't get it finished until two weeks before Christmas. The quality of work they did on our camper and the cost of it was -- unprintable, we are still stunned by the experience!
One other thing, what age is your Navara? Check out The Navara snapped chassis page on Facebook I'm not being alarmist but you need to know. We have spent around £1500 reinforcing our Navara chassis only to find now that Nissan are offering some people new chassis's or a buyback scheme!
If you need more info or pictures I can post some for you.