If you look around Lowes or Home Depot, some of the light fixtures have an 'EZ Mount' system. It's basically two slots that grab on to the original fixture mount screws. You push the strip up and twist it into place.
It solves the hardest part of the process, which is supporting an unwieldy 8' long piece of sheet metal up on your ceiling while you try to wire and mount it. Most of the fixtures have stab-in style wire connections, so the wiring is actually remarkably easy. If you can't find the quick mount system, my recommendation would be that you pre-determine and mark where at least one joist is near to the ceiling box and your fixture center-line. When you are on the ladder with the fixture, mount the middle screw into the joist. Then it is up there without falling on you, you can swing it out of the way, and you can determine your other joist locations for mounting and take your time with the wiring.
Here is my most recent hack job for converting one ceiling fixture into two:

If my HVAC ducting or the knockouts had been in a more convenient place, I would have just mounted one light directly over the box, and run a small piece of conduit between the two lights. But the spacing just didn't work for this part of my ceiling or fixtures.
Don't buy the T12 fixtures, as those bulbs are being phased out. The T5 High Output bulbs are awesome, but hard to find locally. I have three fixtures with 12 T8 daylight bulbs in my garage, and I can turn night into day.
This is so easy to do that I could swing by and do them for you in about 30 minutes if you can't get them done yourself.