I recommend New. There have been vast improvements since the early models. Gearing, Shifters, Brakes, and more. See photo page for examples of each. It is very hard to find used Rhoades Cars. Most often they are located far, far away. Shipping is rarely offered by private sellers. Most of the used Rhoades Cars I have seen have rusted components. You will wind up spending more money trying to fix up your used Rhoades Car that if you just bought new. I have also found that no bicycle shop wants to work on these. Modern day bicycle mechanics are interested in the new high performance 2 wheel machines. They look at a Rhoades Car as a Novelty and have no interest in spending time on them. I have offered extra $$ for mechanics to work on my used one, no takers. So if you plan on buying used, you better learn how to repair you own machine.
36 Speed without question. Great gearing selection, super low granny gears for steep hills or off road. High gears will allow you to pedal down hills up to 20mph. I cannot see any application where a single speed would work. 6 Speed is fine if your on a mostly flat surface, pavement only.
Single Seat is great for the person who won't have to worry about balance, also you have the convenient carrying basket to transport your grocery items and such. Two Seaters are perfect for most applications. Easy to ride solo or with a partner. The framing behind the seats also allow for extra seats that small children could ride in(6 and under). Four Seaters are great for the person who want to earn money transporting folks around the boardwalk or park. But keep in mind, for home/personal use they are much bigger and heavier. Harder to maneuver around. Every ounce matters when pedaling. Adding the extra weigh of the bike, plus 300lbs or more in passengers will make this machine useful pretty much on level ground only.
If your roads or streets don't have an ample shoulder or bike lane, you may have a problem. Transporting the bike is ok if you have a truck or small trailer, and as long as the hassle doesn't outweigh the fun.
My 75 year old parents have been able to enjoy riding my Rhoades Car. As always, check with your Doctor before attempting any physical activity. As far as ease, the Rhoades Car is as easy to pedal as a regular bike. The seating is much more comfortable and the angle of pedaling is easier on my bad knee. You don't have to worry about balancing either.
You don't want to leave it outside, the elements are hard on these, as any bike. I recommend a garage or storage shed.
Pros; Easier to Pedal, especially with hills or long trips
Cons; Cost, 1. adds around $2k to the price. 2. adds Considerable Weight making Pedaling Alone much Harder. 3. Many Areas and Parks do not allow motorized vehicles or any kind.