Plumbing and Piping - Low hot water pressure in older home

My friend has an older home with very low water pressure on the hot side. How far back in the replacement process do I need to go? Replace all exposed or just elbows and horizontals? Replace with copper or CPVC? Which is easier to work with and which last the longest? At elbows what is the perfered transition method brass fittings or specialized unions? Is this a project that I can tackle? I have done alot of sweating (pipes) and have installed bathrooms.
Thanks for your advice,
Bruce

I think copper is the way to go type M will suffice. If you have sweated copper it is a doable project. The pipe in the walls and other spaces hard to get to could be one of the biggest challenges. If you do this run all the lines you possibly can like the main and branches before you shut the water off then shut it off at the last possible moment to connect everything up. As far as what to use. Use copper fittings. Unions on the hot water heater and anywhere else you decide not to replace the copper. To go from Galvinized to copper a dialectric union should be used, They are available at places like Home Depot. I would also reccomend replacing all the pipe if possible if it is so old it is clogged.

Good Luck

As John said, replace all of the galvanized with copper (he's a pro, I'm not), and that's what I would do.
It is notorious for scaling up inside, and eventually clogging completely. That's why it is obsolete and never used anymore. It usually clogs worst at the ends nearest cut-offs.
Initially, you might try replacing the cold water line to the water heater. Cold water pressure into the heater directly affects the hot water pressure coming out.
Again, as John said, use di-electric unions between galvanized and copper.
http://www.generich.com/customer/prodpics/web1566a-1576.jpg
Di-Electric Union
Good Luck!
Mike

One correction to Mike's post. I am not a pro plumber, just a do it yourselfer like you Mike. I always look to Plumber 2000 for the pro advice.