I think you are on the right track by buying either hypertherm or TD.
they are the hands down best machines.
I currently run a Thermal Dynamics Pakmaster 75, which is nominally
rated at 3/4". I find the ratings are a bit overly optimistic, and you
should really overbuy in terms of size. Best policy is to go to a
welding supply store, and actually cut your material with the machine
you want- then you can see how it really does on real steel. My welding
supply store will do this willingly- most will. And most good stores
carry both Hypertherm and TD.
Consumables are, well, consumable. They get used up. And they are
indeed expensive. Life of consumables is directly related to number of
pierces- this is the hardest on them. Theoretically, they would last a
lot longer if you just made one long cut, instead of lots of short
ones.
It is true that a good quality water filter will help consumable life,
and edge quality as well. I like the big black anodized "toilet paper"
style from motor guard, available at most good welding supply stores.

As far as edge quality goes- the damn torch gets hot- 45,000 degrees or
so. So even though the HAZ (heat affected zone) is very small, it does
do a number on any steel that can be hardened. Edges, and slag, get
hard. Slag is the hardest- I always try to knock it off before grinding
or machining, as it is really hard stuff. We use an air chisel to knock
it off the back side, then an aluminia zirconia mop disc on a 4 1/2"
grinder, before we machine it.
Slag production is directly related to proper cutting speeds- at
optimimum speed, slag is minimal, and it just falls off the metal, for
mild steel. At too slow of cutting speeds, slag welds itself to the
base metal, and is really hard to get off.
Most high amperage machines have a control for turning down the
amperage, so you can cut thinner material and still get a nice clean,
minimal kerf.
We tig weld to the plasma cut edge all the time- and yes, we
occasionally notice some bubbles from the edge, but it is rare. But we
do prep all edges first- slag removal and light sanding, usually.
Stainless and some bronze alloys are a lot worse in terms of slag- They
need to be cut in a neutral atmosphere for good edges. So expect more
grinding on them.
At correct speed, the edge quality is better than oxy fuel, unless the
oxy fuel is being machine cut, or run by a true expert torch man.
But plasma too gives you its best cut when machine cut. I use a
motorised cutting machine for my plasma cutting, and it works really
well.