The plan is to use a 55g tank, and about 100W of T5 lighting. I do not want to use CO2 in any form (homebrew too much effort/mess, tanks too expensive).
That doesn't work, you have way too much light to not use CO2. In fact, at that point, you'll most definately have to aim for pressurized. T5 is very powerful lighting.
can't see myself ponying up over $100 for fluorite or eco-complete.
If you seriously wanna go into planted tanks, be prepare to "pony" up a LOT more than that.
I expect I'll have to fertilize, but don't want to be in a situation where I have to do so constantly or risk wipeout of my plants
If you have that much light, you'll need CO2 and ferts. If you don't, then stay low tech. I'm not too sure how far the wpg rule applies to T5s, it is different. Low tech is definately the approach you should take if you want low maintenance, no ferts, no CO2.
Other than flourish and flourish excel (both readily available at the LFS), is there anything I'd have to be adding?
If you wanna go cheap, get dry ferts from greenleafaquariums.com . Are you going to have any inhabitants in the tank?
Is it recommended to cycle the tank first, then add plants and fish, or to have the plants in the tank while cycling, only adding the fish later? If adding the plants up-front, would the ammonia levels involved in the typical "fishless cycle" (5 ppm) cause damage to the plants?
Only hardy plants and fish can survive the cycle process. Fish like comet goldfish and many stem plants can.
This place lists which plants are hardy or not:
However, just because they're not hardy doesn't mean they won't survive it and vice versa. You should just aim in the beginning for plants that are fast growing. You can do a fishless cycle with just plants only. Just find the fast growing plants that are cheap like those in the rotala and hygrophilia family.
The place also tells you the light requirement, since you're stepping down to low tech, you should look around for plants that require medium or less amount of light. Who knows, you might even get away with keeping the higher lighted ones. I've kept UG with 1.5 wpg.