I've replaced intake manifold gaskets on several vehicles (only one was my own, others were for friends). It usually only takes a 5-7 hours or so, as long as you don't hit any nasty snags. They're usually pretty straight forward.
For transmissions, I find that they are usually not worth the hassle of rebuilding. I just take them out and buy a replacement. Done it twice. In fact, I changed my sporty-fun car's transmission from an automatic to a 6 speed manual when the auto gave up the ghost. I wanted a stick from the beginning anyway, just couldn't find a decent one when I was looking to get that model of car.
I've been able to acquire a lot of tools and self-taught knowledge through all the money saved on repairing my own vehicles. Its fun.....but I guess I'm the kind of guy who likes taking things apart to see how they work.
There are some things that I can't do because of the specialized, expensive tools needed. If I want to vacuum down the A/C system and recharge it, usually I let the shops do that. Exhaust work is a PITA, so I'll let a shop do that as it's tough to screw it up. I also haven't really messed with transaxles yet (FWD transmissions) because I haven't had a need to. Depending on the issue, I can learn, but I'm not sure I want to try to pull one down out of the car. The transmissions I've worked with have all been for RWD cars or trucks/SUV's.