eBay seems to be the best economical bet. Online shopping is a reflection of reality and such the typical precautions should be taken before submitting payment to a stranger. I typical will only transact a deal using PayPal funded by my credit card; that combination gives me two level of dispute protection if needed. I have disputed with PayPal before (merchandise not received or not received as describe and have never had a problem getting a refund.

As far as what to look out for. Ask if the LCD screen has any defects. Watch out for cracked antennas. Ask how long it take for the phone to become registered on the network. Anything over 30 seconds could be an indication the antenna isn't performing well. The phone should get a full signal in wide open area (coverage area is Earth). Stay away from units where the seller will not verify if the phone works or doesn't have a return policy.

A phone, extra battery, wall and car charger would make a good starter kit. I recommended having two batteries at a minimum. Extra batteries for the Motorola and iridium brand phones are easy to acquire so don't pass up a kit just because there is just one battery. I've never used more then one battery on my trips but its a nice warm and fuzzy having extra batteries. All of the batteries for Iridium phones are lithium ion and do not have any memory effect.

For service (acquiring a SIM card) a few google searches will return many vendors. You can choose between prepaid and monthly options. Prepaid is good if you will use all of the minutes before they expire - you can rollover minutes prepaid by paying $50 a month which will also give an additional 10 minutes as well. Monthly can be cheaper if you need to have service on all the time and will have low minute usage.

For your intended use I think you would want to consider having three batteries and a solar panel in your kit. Go for something that outputs at least 10 watts that will be enough power to put the phone in a call and charge the battery (in the event no batteries are charged). The only factory solar panels are the Kyocera (very difficult to find these days) for the SD-66k and SS-66k and the Motorola unit (both pictured in this thread) for the 9500 and 9505. You can use a variety of solar panels that will provide a female 12v car socket. There are many choice for solar panels as some units are low powered and trickle charge a battery - which may not be a good choice if you don't know when you will need to make a call with no grid access to charge batteries. For backpacks there are panels which fold up - I'm not familiar with these models.

If addicted to the net some phone models can provide data service (slow) as well.

Last thing is to make test a call before a trip and in your case at a set interval every 30 to 90 days) and check/charge the batteries at that time as well.