Hi-
As a small business owner and the in house SEO guy, I wrestle with the same question all the time. My best advice would be do the things that will get the biggest bang quickly first. For example, work on your page titles. they are an easy fix that can have big effects. I would also look at the structure of your pages, for example are you using H1 tags for your product names on the product pages and are all your images tagged.
Once you've done the "easy stuff" I would just start tackling some of the other items you mentioned one at a time. Page speed is probably a good one to go after, if your site really needs it. That has obvious benefits in terms of visitors. Content such as product descriptions are a slow battle. Things like that require time to make the changes so what I do is allocate some time each day to making them. I track my changes in a spreadsheet and check my rankings here at Moz on a regular basis to see what is/isn't working. A good suggestion might be to look for someone who can write well (an intern) and have them help you with that part.
Even though it's really important, I would wait until you finished some of the other things before starting a link building campaign. You want to make sure you are "link worthy". When you think you are ready, I would create some interesting underwear content and see what that turns up.
Hope this helped!
Ken