When you get the persulfate, it should have instructions for mixing with it. When you order, request the Material Safety Data Sheet, just for kicks if nothing else. It's good to have on hand in case of accidental spills, waste disposal or accidental injestion, etc. Etching capability is related to thickness of the copper as well as surface area to be etched. To maximize your etchant, do conservative board layout, making sure that all areas unused for signal traces, etc. are masked over with resist and connected to ground. That way, you're only getting rid of the copper that connects traces to stuff they aren't supposed to be connected to.
You can always buy more copper-clad board for your next PCB projects. Tinnit powder is good for tinning the etched board, but be aware of the fact that it has a very limited shelf life after mixing. It precipitates out like crazy. Warming up stored solution may help.
Even though you may have lots of second-hand resistors, resistors are cheap, and it doesn't hurt to start a program of buying bulk packages (200 count) of the most-used values. If you're getting into more precision electronics such as A/D converters or test instruments, you'll start wanting some 1% tolerance resistors. You can start by buying smaller packs of these (10 count or 20 count) in values like 100, 1K, 10K, 100K, 1M and then later expanding the values to yield voltage dividers of x2, x3, x4, x5, etc.
Although you may have a good stock of second-hand caps, you never have enough 0.1µF and 0.01µF disc ceramic types ("M" or "Z" tolerance) for digital bypassing applications where you ought to have one cap per chip in your projects.
Dean