Thanks in advance,
Lets do the math. 6-gw is 6 clubs. Each shaft will cost you $10 minimum for steel plus $5 minimum for the grip. A shaft pull and replace around here will run you about $10 per club. A lie adjustment will cost you about $5 per club. Add that all up and you are looking at about $30 per club x 6 clubs is $180. This does not take into account the fact that your club heads are weighted for graphite regular length shafts (if I read your current set up correctly). Replacing those shafts with +1" steel would send the swingweights off the charts. And replacing them with quality graphite would definitely cost a bit more. PGA Value Guide currently has those irons resaling around $180.
My advice is to skip the re-shaft and sell the clubs at an asking price of $250 (read $200). Put that $200 on some new fitted irons, or slightly used 1" over irons and have the lie adjusted. YMMV
I would say any of the major 5 or 6 brands are a safe bet as long as the shafts are fitted to your needs. Clubhead construction is important too. Most opt for a more forgiving head (like the S2) if they cant get out to play a lot, and a more traditional blade style if they are looking to work and improve. You can usually get a large portion of your investment back if you shop carefully and buy slightly used clubs (from a major manufacturer) - that is if you decide to upgrade or change your setup later.
Club head weights vary quite a bit by manufacturer. I do not know of any company that makes two different sets of heads depending on whether they are going to shaft them with graphite or steel shafts. BUT, the clubs can be weighted differently using tip weights to change the swing weight. Typically graphite irons will have lower swing weights than steel shafted irons but this depends a lot on the weight of the shafts being used. There are 115g steel shafts, 90g steel shafts, 85g and 95g graphite shafts, etc...
If you're looking to upgrade your irons I'd go out and hit all the major OEM's. You can save a ton of cash by picking up last year's model. Sell your current irons on ebay.
The heads will weight the same, but the graphite shaft will be 1/2" longer to make the same swing weight. Back in the day heads used for graphite shafts were 10 grms heavier, but the manufactures figured out that the 1/2" would come out the same swing weight., so they wouldn't have to stock double the inventory.