Wear safety glasses and probably ear protection ... You'll thank me later.
Go SLOW.
WATCH YOUR FINGERS!
WATCH YOUR FINGERS!!
WATCH YOUR FINGERS!!!
You should be able to find 78" doors in stock in either Home Depot or Lowes. It would definitely be worth your while to take the time to get them for two of the doors.
If you are working on a budget, buy all of your tools on craigslist. You should be able to get them for 50-75% off and if you decide you do not need them, just re-sell them and get your $$$ back.
Regarding the dust issue. When embarking on renovation projects, you should always be striving to keep your work area tidy for safety's sake. Get a GOOD shop vac and use it frequently. With two people you probably can have one vacuum while the other cuts, which would practically eliminate the saw dust.
While you can do the whole job with just a circular saw, it would be best if you had:
circular saw, use for top and bottom cuts
table saw, use for side cuts.
planer
palm sander
You should have purchased doors without pre-cut knob holes, so, I will assume you did. After you determine the amount you need to remove, divide that in half, then cut off of both sides. This should eliminate the need to modify the interior structure of 3 of the doors. The final door where you will have to reinforce the side(s) where you make the cut, unless they are 6 panel doors, I would remove the whole amount with one cut, then re-use the interior reinforcement or rip down a 2x4 to put in. If you are painting the doors, I would just fasten the structural element with a short drywall screw then cover with compound or caulking. If u r not painting, u need to glue them.
You are correct in stating that this is a relatively easy job, just take your time and WATCH YOUR FINGERS.
Good luck.