Hmm... I admit I tend to not worry too much about soil quality but it does have an impact. But since I'm in it for the long haul I just keep growing and keep adding mulch (wood chips, fall leaves, etc.) which makes the soil a little bit better each year. I just released a blog post on mulch you might find useful. I will be making a Steemit post about it tomorrow.

As far as seeds I find Baker Creek to be a good source for vegetable seeds. They have a lot of options and some rare/old varieties.

Good luck!