I would certainly have a shop with a well proven track record build the block. There are lots of Subaru specific quirks and such that your everyday mechanic won't know. As far as everything else (R&R engine, suspension & brake work), you could do it.
I my experience, your car will do plenty of sitting even if you have all the money up front. I've always paid for everything up front and I've only been able to drive my car for about six months in the last three years.. for example, this year I made a plan with my builder and paid for all the parts in April and scheduled an appointment to have the work done. I dropped my car off May 14th under the impression it would be ready three weeks later. Months went by, calling the shop every week, they were about to get started on it "next week". Finally in August I started calling multiple times a day, asking a lot of questions. Eventually I got word that they did not have the cams I ordered and apparently couldn't get them. Over the next week I called probably two dozen suppliers and shops across the country. I came to find out that my cams had been on extended back order since March 1st, and GSC doesn't expect to get their next shipment of blanks till the beginning of December. So even if you pay and schedule everything as well as you can, that doesn't mean your car isn't going to be sitting for several months at a time.