How much to spend?
First, determine what you plan to do with your rig. I’m primarily a hiker, so I modified my 4Runner to allow me to reach areas where I want hike. That meant a lift, larger/more aggressive tires, and rock sliders. This also allows me to explore somewhat remote tracks, thereby satisfying my inner Indiana Jones.
What do you need to add to your vehicle for living? A rooftop or ground tent? Awning? Storage? Bedding? Kitchen setup? I bought a two burner propane stove, which is more versatile than my backpacking stove. I replaced my stock roof rack a full length version - a place to store items that may not fit inside the truck. I also use it as a photography platform and sometimes just a spot to sit and watch a lovely sunset.
What accessories will you need? Definitely some maintenance items/tools, recovery gear, maybe a communications setup. I added a console safe to secure valuables and made some insulated window covers for privacy. The more you repurpose from home or get on the cheap (Craigslist, eBay, discount stores, etc.), the easier it is to afford those big ticket essentials.
Build a spreadsheet, research prices (some are quite shocking!), set a budget and start with what you consider most beneficial. Save up, pay CASH for each upgrade, don’t put yourself in debt. Heck, you may lose interest or find that overlanding isn’t your thing. You’ll also learn what suits your style and this will influence future purchases. At first, I thought I wanted a RTT; after much thought, I purchased a teardrop trailer. I still use my ground tent for some trips, but the trailer is sweet for a base camp.
Whatever you decide, it should be for YOU to enjoy and not to impress others. I’ve never been cool, but I’ve had a helluva good time being uncool!