I've found that with few exceptions, most minor to moderate home repairs can be accomplished more quickly and far cheaper by reading up on it a bit, maybe watching someone do it on HGTV, then figuring it out as you go along.
By the time I get a contractor to call me back, get them to come out and look at the job, get me an estimate (which is never what it eventually costs, usually half to two thirds), deal with them showing up late or not showing up at all, doing half the job and then disappearing for two weeks, then doing a half assed job...its far easier and takes less time to just do it myself.
For jobs requiring special tool(s), I sometimes rent them but often just buy them, then I have the tool forever and can loan it to other morons who buy stuff they'll only use once or twice in exchange for their oddball tools
This is easily rationalized by noting that the cost of the tool is cheaper than the cost of paying someone to do the job.
I used to get wasp nests in my attic all the time. Unfortunately, they built their nests inside the blown in insulation and there wasnt any access from the attic side to spray them. $2000 was the quote to remove them.
I ended up using an electric leaf blower and about 25' of pvc drain pipe. Stuck the pipe into the intake 'mulcher' side of the blower and duct taped it in place. Tapping on the side of the house near their entry to the attic, I managed to coerce almost all of them into coming out and buzzing around in a nice dense cloud where I vacuumed them up and made wasp mulch out of the lot of them. Returning the next morning I gave them a second round which all but eliminated the workers. I then turned the pipe around to the blower side, stuck the pipe up next to the crack they were entering through and dumped a cup of Sevin insecticidal dust into the intake hopper. That coated their entry and probably blew a quarter cup right into the nest where they all died within a few hours. That winter I'd go up in the attic and dig up the nest. One was a couple of feet long, a foot wide and more than six inches thick.
Since I had to do that a bunch of times over 7 years, I'm glad I figured out how to do it with $30 worth of drain pipe (which I also used in a similar hookup with a 'u' joint on the end to blow leaves out of my second story gutters), an old electric leaf blower and a buck worth of bug dust.
It was also somewhat fun, and thoroughly amused my neighbors.
As far as the other stuff, as far as my experience runs anything thats small enough to come out of a duct will come out on its own and anything that isnt wont. I usually get a flexible vacuum hose from the shop vac and vacuum out all the registers back to a foot or two. Generally all I get are nails and bits of wood leftover from the original construction. When I've removed ducts, the 'low spot' in each duct run usually has a whole bunch of nails, small kids toys and other jetsam. Nothing that was going to blow out or cause much of a problem. Most ducts that were 15+ years old had a coating of greasy dust in them that could be removed but would likely be quite quickly replaced by more greasy dust. As far as whether this greasy dust was allergenic, had mold or dust mites in it...well...the average carpet, sofa or mattress is a far better and more prolific breeding ground for these items.
Only thing you have to watch out for with water softeners is the salt speeding up the corrosion of iron pipes and the nibbling loss of copper and solder due to mineral induction by the softened low mineral 'hungry' water. Soft water wants to absorb any mineral it can come in contact with. My old house had a softener and very hard water coming in...many of the pipes had a little blue 'dribble' at the joints. Probably not a problem in a well installed plumbing system, but if you have a few flakey joints or a thin pipe somewhere...
Toilets? Pay under a hundred and you're not going to get a very good toilet. Costco is selling one right now for about $80 and it looks pretty good, for a lone violation to that rule. Generally at about $180-200 you're getting something well made with good flow and long life parts. Clean all of the old wax off of the drain, let it dry fully, make sure you put a rag into it while working so you dont breathe any sewer gas, seal up any cracks/seams and the area around where the drain meets the floor with good silicone caulk, let that dry over night, use the extra thick wax ring WITH the plastic 'horn' on the toilet and you'll have little trouble from that ever again. The hardest parts of this job are any rusted on securing nuts, which you may have to cut with a grinder or saw, and picking the thing up and putting the new one down.
Speaking of which, I have one downstairs that moves about 1/4" every time I use it...which probably means that when they took it up to replace the linoleum a few years ago they reused the wax ring or didnt put it on right and I've got a couple of square feet of floor AND a toilet to replace :P