I just realized how rude i was in my last post. Bad morning, you know ho it goes. But enough with the mushy stuff.
First off I noticed you haven't been here that long, so welcome to marlin crawler. Its a great place to learn about Toyota's and such.
Its long but I hope it helps. Good luck
If you have been into off roading for a while you would know that a solid axle is much better for extreme wheeling than IFS.
Though IFS is not for every buddy, it can get you along ways if you know how to drive it and it is built to you type of wheeling.
If you still want to keep the nice ride and handling quality of the IFS but want more travel, Total Chaos has some good packages for about the same price as a solid axle will run you, give or take. Although their kits are intended morw for desert racing, they can be used for trail cruising just as well.
A solid axle enables you to take on off camber terrain at slow speeds flexing over obstacles.
As far as going with coils up front it would be sweet but it will cost you up the
For all the links and heims. coils are cheap but good, strong heims are
if you are sceptical on wether you can do a SAS with leaf springs, don't even think about coils. They are a better ride and flex way better, they take a lot more work to make fictionally strong.
With your 3.0l I am not sure, I know My buddy just put a Chev 350 in his with a 3" lift and hit drive line hits the oil pan under flex, maybe somebody else can answer that one.
Quality leaf spring (such as Marlins) are going to give you a good ride, the important thing is the shocks you choose and how they are dampend that will determin your ride quality, also tires. A mud terrain or radial is like night and day over a swamper, the ride is way smoother and quieter with mud terrains or radials. But you gotta love the humm! of a swamper
Your frame will be ok, but for security It never hurts to brace the font for steering. Your rear is allready coil sprung from the factory so you can find some sweet kits to give you made flex, but yes they will run you more money do to the complexity of the coils.
Just take you time and measure stuff out, it always helps to have there or four people their to point out something that you may have missed, And you always have the forum to ask questions. Just don't start the project until you have most everything to complete it.
An 85 would be a good platform to, You would have the runner as you r daily driver and the 85 as a wheeler and not have to worry as much.
But the runner SAS would be way cool!
Hope this helps