No advice on a specific mountain bike frame. I suspect they all will work just fine. Some might recommend steel since it can supposedly be repaired easier because you are likely to run across people in the boonies with a welder for steel but not a more specialized TIG welder for aluminum. For the most part this is wishful thinking. The cheap welders you will run across in the boonies are the ARC stick welders owned by untrained people who weld broken agricultural implements using heavy welding sticks. If you tried to weld thin steel bike frame tubing with such a welder and untrained person, you would melt your thin steel tubing.
I've often thought of converting my mountain bike, Raleigh M600, to drop bars for loaded touring. This is what is required, assuming you are starting with a relatively new model Shimano equipped mountain bike.
1. Rigid fork. I don't want a suspension fork unless its very rough trail. The fat tires will provide enough cushion with a rigid fork touring on gravel or dirt roads. $65 Dimension steel from QBP or $137 for Kinesis aluminum from QBP. See Harris Cyclery for the QBP listings. Any local bike shop can order from QBP.
2. New headset race for the rigid fork. Or you could take the race off the suspension fork. $5
3. Drop handlebars. I like my Nitto 115 drop bars. They are 25.4 diameter so they fit MTB stems. $30 from Harris Cyclery. Or you can get 26.0 diameter road bars and then replace the stem cheap enough. You might have to replace the stem no matter what depending on where you want the reach to be.
4. Bar tape. $5
5. Maybe new shift and brake cables. $20 or so.
6. Shifters and brake levers. Two choices. Bar end shifters. 9 speed for $80, 8 speed for $40. Brake levers Dia Compe 287V for $60 from Harris Cyclery. These brake levers work with V brakes so no adaptor is required. OR go with STI. $130 for Shimano 105 or $160 for Ultegra. Plus two Travel Agent V brake adaptors for $20 each. These allow the STI levers to pull enough cable to work with V brakes.
About $250 to $350 depending on how fancy you want to go. If you have not bought the mountain bike yet, then you can switch out the parts at the shop and maybe pay no difference.