i replaced the exhaust manifold on my old jeep w/ a header. it was a pretty big job, & it took me a long time, mainly 'cause it was the first time i had ever done it, & i was working alone. i followed the instructions in a haynes manual that i had. if you have all the right tools, don't run into any major problems, & have someone helping you, you could probably replace the exhaust manifold in one full day.
i've never replaced a head gasket, but i imagine it would be at least as much, probably more, work than doing the exhaust manifold. if it is a blown head gasket, you should get it fixed right away.
if you have the time, basic mechanical skills, the tools, & the interest in doing it yourself, it is certainly possible. whether or not it will be worth it is up to you. it was worth it for me to do it on my old cherokee 'cause i learned a lot in the process, it was satisfying to me to do it myself, & i saved a lot of money.
i probably won't be doing much work on my '97 lx450 on my own until the extended warranty expires in '06. the stakes are higher w/ this vehicle than w/ my jeep.
good luck.