It is good to see that you have been listening, it is something that a lot of us did not do at your age and that is why all of this advice is so valuable. Walking is not as fun as driving. At your age and capitalization, do nothing to a vehicle that starts and runs just fine every day unless you have two of them. Save your money and make plans for upgrades that agree with the content of your wallet.
Head gaskets are reasonable, as low as $43 each, or $95 for a valve grind set, but you still have to replace the intake tin, which is another $45. 6.9s are not real powerful, but they last a long time and are plenty durable. Patience is your best friend right now.
And to answer your first question, yes it is possible to change out head bolts for studs one at a time, it is just not advisable (and not necessary if you are not adding a turbo immediately). See reference to multiple vehicle insurance, above.
Good luck in your future endeavors, you have chosen wisely by asking your questions of this crowd.