You're on the road to a very nice lawn, and have received some very good advice. If you have a load of fabrication experience, building a topdresser might just work out fine. I don't, so I bought an "experienced" Turfco Mete-R-Matic on eBay and rebuilt it. It was a rusty pile of junk. I took it apart, figured out what was truly broken, wire brushed the whole thing, painted all the parts, bought what replacements I needed and put it all back together again. I now have an $8000 topdresser for a little under $2000 (counting something for my time, as well). Topdressing isn't something you do once or twice. It should be part of a continuing program of sustainable turf management. I spread composted yard waste and coarse sand on my lawn at least once a year, and twice when I can afford it. Another couple of years, and it will look like a fairway. It already (in 3 years) is the best looking yard for several miles in any direction. Topdressing has cut my lawn's fertilizer requirements by 2/3.