Let me get it right, you want two weeks adventure for two people, budget is 50k (max).
You want to see countryside, and do it by bike...

Why to buy a cow, if you only want to drink milk?

Ever though about joining a small group? Their are many organised Groups (usually max 5-10bikes), but the operators mainly use CJ750 sidecars.
They head out into the nowhere and you dont need to worry about fixing the bike, ordering food and get into the a foreigner accepting Guesthouse.

50k RMB is plenty enough for such kind of trip, 20-30k should do for two, usually they are organised around 10-12 days.

I only took such a trip with Yangtsepete, www.yangtsepete.com - just check with him, if something available to suit your schedule.
Otherwise he maybe can refer you to somebody else or somebody knows other operators. I definetly can recommend his product!