50mm on a budget: pretty easy. You're right to avoid Soviet lenses - they'll all need shimming to focus properly. Early Canon LTM 50mm lenses can be had cheaply and perform well. The Leitz Summar and 50mm f/3.5 Elmar in LTM are both quite cheap. Watch for haze in all of them, and especially in the Canon lenses (doubly so if they have black barrels) - Canon's lubricant of choice formed a haze that etches the glass.
Sometimes you'll get lucky and find a good deal on a 50mm with a rear lens cap in the form of a film body, too. That's how I ended up with both of my Canon 50s. Do I want the bodies? Not really. But they're nice to play around with, I guess.
28mm on a budget: a bit tougher, as there's not a lot of old 28mm lenses going cheap. The Soviet Orion-15 is hit-and-miss, depending mostly on the factory it was made (check the logo), and is slow. Pretty overpriced now, too. Canon did some good LTM 28mm lenses that would be the better option. There's a lot of good options from Voigtlander in both LTM and M, but expect to spend 1.5x to 2x as much compared to a Canon 28mm for even the cheapest ones. That said, if I didn't already own the earlier f/3.5 version, I'd go for the new f/2.8 28mm Color Skopar; the LTM version is on sale for $549 at Cameraquest at the mo, and that is honestly a bit of a bargain considering how good it is:
Voigtlander 28mm f/2.8 Color-Skopar Aspherical L39 Type I Silver: shop.CameraQuest.com
There's also some new (or new-ish) options out of China that are reasonably priced, like TTArtisan's 28/5.6 and 50/1.4 lenses, or Thypoch's 28/1.4 and 50/1.4, if you're willing to spend a little more. I've had a play with Thypoch's 35/1.4 and it looked pretty good. I wouldn't call those "budget", but they are budget compared to Leica (or even Light Lens Lab).
