Sounds like you have been doing some research already....good for you!!
A few lessons learned that some of us have painfully gone through:
It costs almost the same to fill a 5 pound cylinder as a 10 pound cylinder. You figure out the cost benefit. I have two ten pounders, once for travel and one stays wit hthe kegerator. When one is depleted I have a backup (came in handy at deercamp to have the spare).
I really lost my butt a few times when starting out because of the seals on the keg not being 100% sealed. Leaving the regulator open to feed my taps was what I wanted to do. A trip back to the gas place two days later with an empty tank happened more than a few times for me. To fix this problem, I never leave my regulator open for long periods of time. If the kegs stay pressurized when left unused for a few days then you will be OK and shouldnt waste gas.
I recommend replacing all of the o-rings on old kegs you buy. I have replaced every o-ring on all of the dozen or so kegs I have and it was well worth it, it helped solve the problem I had mentioned above.
As for carbonating my beer, my SOP is to hit the beer with 30 PSI as soon as I keg the beer andc then let it sit in the fridge for a few days. Thenm hit it again. When I am ready to pour the beer I releae all gas and hit it with serving pressiure. It is NOT scientific, but being and engineer I get tired of being scientific sometimes and just go with what works well for me.
For filling the kegs, its easy. I only use the 7.9g plastic buckets with spigots. I clean the keg and fully fill with Co2...purging all the air. I attach a drain hose to the fermenter then drain out the first two pints, they will be yeasty. Then I snake the hose down into the keg and fill from the bottom up, pushing out the Co2. It never touches air, I even don't disturb the Co2 blanket on the fermented beer when its draining.
Hope these and the other comments give you some new ideas.
Cheers and beers!!