Chevy & GMC Vans

I just purchased a 2002 Astro AWD, with 60,000 miles. A friend whom I bought it from, has owned it since 2003 when it had 15000 miles on it. He just used it for the kids hauler or getting this or that. Hence the low miles.
Recently, he has had some trouble with the coolant disappearing, and decided to get rid of it. I got it for a song and a dance, and don't mind sticking money into it. This is my 4th Astro/Safari with a total combined mileage of 870,000 miles.
I asked my friend about the coolant, and this is what he said. A couple weeks ago, when it was cold, he started it, and noticed it putting out a huge cloud of white smoke. He also said he had been having problems with the heater, where sometimes it would throw out hot air, and sometimes not. He kept adding antifreeze, but can't seem to guess where it's going. There is no antifreeze on the dipstick, and the oil isn't over full. I've read articles where either the head gasket or intake gasket goes out on the 4.3 engines.
My friend told me he took it to a performance shop, and they tweaked the ECM to give it firmer shifts in the transmission, along with an engine performance enhancement.
I do know, he has a heavy foot, so I'm guessing, even though he did good maintenance on the engine, he was not light on the loud pedal.
Today, starting it up, to move it out of the way for snow cleanup, it started right up, and didn't smoke. Even after warming up.
So this is what I'm asking. What should I look at, the head or intake gasket? I'm willing to do either or both, but would like some insight as to what or why these gaskets fail. Is there anything special I should do to be sure this doesn't happen again.
Come to think of it, my first Safari (95) with 318,000 miles had the same symptoms before I retired it. I never did look in to seeing what was wrong.
Thanks to any and all answers.

As someone who is tracking down the source of an oil or brake fluid leak, I think your best bet is to find the source of the leak 1st, its really the only way to start. Rather than replace a gasket when the only indication that anything is wrong is a low coolant level, give your engine a cleaning, check all your hoses, take a look by the manifold for wet spots, check by your thermostat as well as drops can pool by it. You mention not getting heat, the thermostat could be stuck. You can test if the thermostat is working by letting your van run for a few minutes and seeing if the top radiator hose gets hot, which would indicate the thermostat is opening and letting the hot coolant through, if it is cold it is probably stuck, and is a fairly doable/cheap repair for that portion of your problem. (It will not open until the engine is hot, so give it a few minutes)

If it is your gaskets it is an involved job, but also doable, I just don't think you should dive right into that one until you know its bad.

Good tips Wiley. I would spend some time as well wiping things down and looking for the source of the leak. I would not suspect a head gasket as them failing is pretty uncommon. Intake gasket leaks are very common though. Pull the doghouse off and look for signs of a leak near the rear of the intake. Also look at hoses, bottom of radiator if you live in an area where the salt the roads, weep hole on bottom of water pump etc for any signs of leaking. Run the van for a while, keeping an eye on the fluid level until you know for sure where it is. There are several threads here about this same issue.