1) PartsRack has two types of tenders for $59. The onboard, fuel/oil/spray resistant installed unit that stays with the car......and the Deltran unit that can go car-to-car.
Both have auto shut off, fully automatic float modes. [NEITHER are Exide ..... I'd appreciate your order, thanks.....]
2) Starting the car every 30 days MAY be a good idea, but only if you can get the oil up to operating temp to evaporate any moisture created. The plus side is lubricated seals, gaskets, and moving parts. The slight downside is a bit of impurities in the oil. I run mine to temp 1 x 3-4 weeks in winter months.
3) I much prefer the DelTran removable Tender.....the Exide mount-on makes the car "look like it has a problem" to the casual observer, and then cannot move car-to-car easily. But it is the cheapest.......we don't carry it. And there are $25 cheaper units elsewhere.......