visas and loi's
we rode the stans south to north earlier this year on belgian passports and didn't need any LOI, except for Iran. we got a second turkmen transit in dushanbe, also without LOI, on the way back. i'm not sure if it works the same if you apply in teheran, but we got all our visa with no trouble and no agency. and here's another idea: you can apply for visa in one country and pick'em up in the next; we applied for the turkmen one in tadjikistan and collected it in uzbekistan. no delays, no hassle.
good luck and enjoy, it's a lovely place