You would want to have some locomotive tracks with wide radius curves for the big boy to use to hook up to the train. You would not want to use a big boy for switching anyway. Another trick to use to maximize your yard space is to use "wye" switches. I think a #3 wye is the same radius as a #6 switch, but only takes up the space of a #4. In other words, make your yard as tight as needed to make it work for you. 0-6-0, or 0-8-0 switchers, or even an Alco S1 or Nw 1 will handle 18" radius easily. and the diesel switchers will probably handle 15" without problems. The only yard needing wide radius turns would be a coach yard if you are using scale 85' passenger cars. Then put in some arrival and departure tracks adjacent to the yard with wide radius to handle your mainline power. Another idea to hold down costs is to remember that scenery is relatively cheap. If you emulate some of the modelers on this site, and build structures from cardboard.
Then build a layout with a high scenery to track ratio, you can probably build it for a lot less than you think. Regarding code 55 rail, I think anything with rp25 wheel flanges will work fine. It might look good to use code 55 in the yards and industrial sidings, with code 70 or 85 on the mainlines. You might also reduce costs for a big layout by getting enough track down to get trains running, and then try handlaying turnouts. Generally the turnouts are the most expensive part of track laying, if you can hand lay them, you exchange "sweat equity" for turnouts that would otherwise cost $$$.