You have done well to get a solid deck 45 series as your 1st cylinder mower. I have a 17"(cost $300) and 20" (cost $200), both are twin rail mowers. Both have cracks in the rails, but the 17" are only small at the moment, and are only on the topside of the rail. The 17" came with a Chonda GX160 and 20" came with a B&S 3.5hp. The 20" had some flat bar welded to the rails front and back like the one you are looking at. It also has some round bar welded on the inside of the rail between the top section and the shorter middle section. This indicates that the rails have probably totally failed, as the cracks have moved from the top side of the rail and gone through both sides. Having inspected both of my mowers rails I decided to put the lighter and less powerful B&S engine on the 17" and use it this summer. I am now rebuilding the 20" as I have to replace the rails, then I will fit the heavier and more powerful Chonda GX160 to the 20".I have cut down some 3mm thick 50x50 RHS to replace the rails on my 20". This will make the rails stronger and slightly thicker than the original bent steel plate rails. When the 20" is up and running, I will rebuild the 17"....... Well that is the plan so far.
To answer your question about the 17" twin rail for $500, it does sound a lot. But if the catcher, reel and engine are all in very good condition it may be worth it. As these 3 items are the most expensive parts of the mower. As they all can cost between $200 - $400 each, to replace.
