One problem with slide projectors on Internet auctions is that the shipping can be awfully expensive. You may check thrift stores, garage sales and estate sales for projectors, where you will not be paying for shipping, and can test the projector to see if it works.
I haven't had any problems with the Carousel 140 trays, but be aware you can't use the 140 on the first generation carousels (the 550 being the only such model in any abundance).
Note that the Carousels with three-digit model numbers, but no "H", use traditional tungsten bulbs, while those with the "H", or a four-digit model number, use quartz-halogen reflector bulbs.
Don't disregard the projectors using the Sawyer's Rototrays (the tray that stands up like a Ferris wheel), they are reliable (assuming the slides are straight and intact), and easier to repair should the projector itself have a breakdown.