Use only OPTI-2 fuel additive on 2-strokes. You'll NEVER have fuel problems (along with mid-grade gas).
Buy from an independent dealer. Get a good quality machine.
I go through plenty of line. I'm not an LCO but have a huge yard (15,000 sq/ft lawn on a bigger lot). I use ROUND-UP to get some areas several times yearly (fenceline, street, sidewalks, driveway) to cut time and string use. Line is so cheap as to be a meaningless cost. I may use quite a lot at the beginning of the season as weeds proliferate everywhere, but little by now (June 2).
Once the lawn/yard has been "shaped" (to accommodate not having to pull mower backwards; 90-degree corners cut off, etc. Mower should never have to stop, IMO) then weedeater time is cut to a minimum. Mine is used nearly every week of the year for one to two hours at a time (haven't built bed edging yet, so most is a variety of "edging" to keep a straight razor-cut on lawn edges).
I've gotten eleven years from a cheap machine that should have [censored] out after 3-4 years. Can't bring myself to get rid of it.
The use of the machine is an acquired skill. I've been using them since the early 1970's (about cried from joy as doing work by hand with clippers was a killer).
Train yourself. I always have an angle-of-attack on mine, never perfectly horizontal or vertical. Bermuda requires one technique and St. Augustine another . . . I move head right-to-left and prefer to walk backwards using peripheral vision to see ahead and behind of line sweep. (Right handed, but left eye dominant). You'll find what works for you.
As to chain link, just turn it vertical. Angle head in towards fence bottom. It ain't rocket science. On an overgrown fenceline it will take a number of passes (imagine 4-6' Johnson grass, huge ancient clumps at ground level), and working both sides of fence. Have several spare reels of line at all times.
Use a drain spade or half-moon edger to get all growth away about 6" on both sides so line of lawn can be cut so that mower wheels drop to bare ground without snagging on fence. Fill holes, compact, treat. Only hard the first time.
Use commercial grade ROUND-UP to treat (barrier) or learn the use of soil sterilants to avoid this problem in future.
Goatskin gloves are the best long-term. Use GOJO or other to clean them occasionally.