Joining this thread really late.
FWIW, I've seen enough proof of people scraping things flat with scrapers made from old files or whatever that I'm convinced you can eventually achieve success with darn near anything — with enough pain and persistence. I value efficiency more than frugality. so I splurged and bought my hand scraper and blades from Dapra (I've subsequently lost my old Anderson-style scraper).
Personally, I love being able to swap blades between my hand scraper and power scraper. That is a big win with the Biax-style hand scraper design (Keith's drawing).
For the blade, you can get by with a chunk of high speed steel. It's even better than carbide for some materials. Carbide will go a LOT longer between sharpening on cast iron than HSS, though.
I think the Sandvik inserts come with sharp square corners. You really, really, really want a radiused edge (typically a 6" radius) and you absolutely need to grind off any sharp corners even after radiusing or you'll almost certainly get nasty scratches eventually (ask me how I know). Easy enough to build a holder for Sandvik inserts, but you've got to have a way to grind the radius and sharpen the edge of the inserts.
A green wheel will let you do the rough grinding to create the radiused shape, but sharpening carbide well is tricky. You want about a 5 degree bevel. A Glendo Accu-Finish slow-speed grinder makes this trivial, but even used prices for a Glendo are through the roof. It is possible to sharpen by hand on a diamond plate like woodworkers use (I've done it) but it's difficult and hard to acquire the knack. You put the blade in the hand scraper, put the handle up on your shoulder, then with the diamond plate flat on the bench you tilt the scraper just off of vertical and sort of sway your body back and forth to polish the curved edge at a five degree bevel.
If you've got access to a diamond wheel and a T&C grinder, you might be able to cobble something together that would work, but I suspect the higher speeds (compared to a Glendo) might make it hard.
Personally, I'd be on the lookout for used Biax blades even if building my own scraper. If I only had one, it would be the 6" long 20mm wide 150mm radius blade (p/n 20-150). 20mm is wide enough for general use, but still narrow enough to get into at least larger dovetails. New, they are about ninety bucks (far from cheap but worth it to me). Even if you buy it new, you still need some way to sharpen it when it gets dull.
To net it out, the entry point for scraping is an old file, torch, green wheel, oilstone, and lots and lots and lots of elbow grease and patience. A faster path to success is to acquire a Biax blade, build or buy a hand scraper holder for it, and build or buy a Glendo style slow-speed grinder.
Finally, Richard's class is way more than worth it in my opinion. On your own, you can eventually learn to scrape something pretty flat, but it will likely take you a LOT longer than his five day class. And just getting something flat is the easy part. The hard part is learning how to make precision surfaces coplanar or perpendicular, how to measure and correct (step scrape) things that are wrong, and how to apply this specifically to machine tool reconditioning. All of that is covered in his course. Most of it is also in the Connelly book, but you're a better man than me if you can make sense of it (and stay awake!) without taking Richard's course first! <laugh>