If I had 40 grand, I'd probably spend about 15-20K on a somewhat used, but still newish rig (prob a Toyota landcruiser or Tundra). I'd shoot for less than 10 years old, less than 100K on it. I know, my standards for "new" are probably pretty low. But honestly, I like the rigs from that year range more than I like brand new ones.
I'd spend like 3 grand on some nice-ish camping equipment, and spend like 4K to do the super basics on the rig-suspension, tires, keep stock wheels, roof rack, maybe a simple driving light, floor liners, and sliders. I'd hold off on any further big ticket mods. If you're definitely planning on being remote and off the grid for more than 4ish days at a time, maybe a fridge.
If you find you hate overlanding, or you hate at least part of this, or you hate your rig that you bought etc, you won't have wasted 40K on a built rig that you will lose a ton of money on. You can sell your rig, losing some of the money you put into upgrades, and sell your camping equipment losing probably 50%. Way less risk if you keep the initial buildup simple and straightforward.
I'd also choose to keep the sleep system simple. No rtt, teardrop, or expedition trailer. Personally I'd spend some good money on an easy to set up "swag bag" aussie style (assuming it was me doing trips on my own), but that's just me.
I'd spend the rest on the wife or give it to her to spend on the house. Or just save it for repairs and further upgrades once I had a better idea of the things that would bring the most value to my style. Honestly, what I'm laying out feels super extravagant to me, but given you have a 40K budget and are looking to treat yourself, it feels like a good middle ground to me. I've just noticed I often don't get increased value from spending more money on stuff, and especially if you are starting out from scratch, its good to start slow. You just don't know what you're going to find you hate, and what you really enjoy doing. It's easy to get starstruck with all the fancy gear and setups people have, but the truth is most setups that are getting hard use have been developed over years of trial and error. No sense in spending all the money up front before you start experimenting, finding what works and what doesn't is half the fun.
