One thing that hasn't really been discussed in here is what *sort* of Explorer (or pilot, even) the OP is. That has a big effect.
If you take a lot of chances with an SRV, and would be bored to tears driving safely, maybe a size 4 hangar with 2 buggies is better.
If you're a careful pilot, you could probably dispense with the AFMU entirely, or stick with just one small one.
Are you careful with plotting your courses? A stock fuel tank is plenty, so long as you don't blindly let it run dry.
Are you the sort who gets nervous if you hit 2 unscoopables in a row, and immediately open the galaxy map to look where you can take on fuel? Then you could squeeze a little more jump range by dropping *down* to a 16 ton fuel tank.
If you have a history of face planting into stars, take a few heat sinks. If you can count the number of times a star has sucked you out of supercruise on one hand, then maybe you don't need any.
There are a few tricks you can do, too. You can depower your AFMU to fit a smaller power plant, thus extending your jump range. If you need to repair anything, just depower your thrusters (not in flight!) to free up enough power to run the AFMU. You can technically depower your hangar, too, except when you're on a planet. I ran that way for all of Distant Worlds.
Here's my current build, by the way. It's a fairly standard, conservative explorer's build. I got a decent roll on a grade 5 engineered FSD, so the jump range stands at just under 50 light years with a full tank and no cargo carried. I have a large cargo bay carried so that I can do plotting tricks. I can use the cargo carried slider to shorten my jump range in the core (if you don't know what this means, don't worry about it), and to go into a semi-economy mode if I want to do a surveying route through an area.