The 56K dial up is the cheapest option but obviously isn`t particularly quick. It is also the most reliable for your situation. I use GPRS via bluetooth on my mobile phone when I have no other option - it is expensive though for anything more than occasional use because you pay for the download (viewing) of every page in MB`s. For example: click here
Right at the bottom "O2 Active via GPRS is charged at rate of £2.35 per MB. 1MB is equivalent to browsing 150-200 pages, dependent on content. Minimum charge per page 1p."
I find it is difficult to relate to this tarriff because it is dependant on content. The last time I measured it on my bill (Not on the above tariff by the way) it cost me £2.95 for half an hour surfing. I suggest you check out other providers charges.
One other concern is how remote are these village halls? Can you guarantee mobile signals in all locations for each of your advisors mobiles?