Car-Rental Upcountry: Which Internat. Company - or Small Local Renter?

Hello, I will be based in Nan, N-Thailand (because I want to). There are the car-renters Hertz and Avis, plus several local car-renters.

For car-rental world-wide, I have my preferred European agencies (with best prices and very transparent insurances in my native language, good experiences so far); but they don't offer cars from those companies in Nan (while they do offer other upcountry smalltown). So where should I book or rent:

• online straight with Hertz or Avis?

• online with some agency like rentalcars.com? You recommend a another/better online agency?

• go to small local renters and shop around?

What I want:

I need good worry-free insurance. I'd love reliable assistance in case of puncture etc. I want a small car like Toyota Yaris or even smaller. No off-roading at all. 3 – 6 weeks. A deductible is ok if it brings down the daily price.

Hertz in Nan offer a small car i'd like, and not only a uselessly big ship. Avis says, "sold out". Herz also offers one-way rentals without mark-up – good.

They say I could/should buy additional insurance locally. What does that mean?

Or would you recommend using a small local renter? (Link to Google Map Search for Nan car-renters) I had local renters already a few times in Nakhon Si Th. and Chiang Mai. Good experience, but I was a more worriless guy back then. I speak basic Thai, but I couldn't say "loose rear mirror" or "broken indicator front left" in Thai.

Renting from small local car-renter - pros

• In my experience, return day is very flexible; return about anytime and pay the exact days of actual use (I dislike the totally unflexible timing with the big companies)

• Can take a look at the car, maybe get "insider information" about vehicle, perhaps arrange extras etc.

Renting from small local car-renter - cons

• Don't understand all insurance and procedures

• Not sure about quick emergency assistance on a remote mountain road

• Maybe I have not enough Thai language (it's ok for small talk and tourist things)

• Not sure if cars are availaible when I arrive 4 days before car needed; not totally sure if car will be kept for me as promised

• Some seem to operate through Facebook only, which I loathe

  So what do you think? Thanks!