DIY or prefab is more of a question about your available tools, skills, time, and costs. For me, after almost six months of sorting this out, the costs for anything DIY was higher although I had the tools, skills, and time. The materials just weren't available to me for reasonable expense. I also looked at the Wike DIY Trailer locally and was not satisfied given the cost for what the trailer actually offers.
I scored an immaculate, 1-month old, high quality child's bike trailer for 70% off new price on Craigslist, put a more solid base in it, and can use it for ~100 pounds although I can't think of much that I would possibly put in it that would actually put that figure to the test. Groceries--check, hardware store--check, monthly household goods restock--check, cooler/sporting goods/picnic items to the park--check, smaller building materials--most except masonry items, animals--check (I assume 80 pounds of dog could fit in a space previously occupied by 90 pounds of kid, although I don't know whether your dog would agree or not). And I only take the trailer when two panniers aren't enough. For me, switching between a trunk bag, one or two panniers, and a trailer offers enough options to handle everything I would do with my bike for the past two years or so.
Regarding (3), if I had a regularly occurring (at least monthly) need for a more heavy-duty trailer, e.g. masonry goods, long boards, multiple storage bins, really large items, I would not hesitate to buy one in addition to my weekly go-getter. Where road/mtb cyclists get excited about the n+1 bike, utility cyclists can easily get amped about the n+1 trailer. Just make sure the need for "heavy duty" and "larger capacity" is really a need. You do remember that you have to pull this trailer, right?
