If you can sell either soft serve, or frozen carbonated beverages, you will make lots and lots of money. The profit margin is huge! Call your local Taylor dealer and take a look at their used equipment, it really isn’t that expensive.
those that say free equipment will cost you something in the long run just haven’t, imho, taken a hard look at some of the deals out there - brand placement and recognition is so important and valuable to most well-known food manufacturers that they are more than happy to provide you with equipment with their name pasted all over it. it’s a win-win sitch for everyone - you get free equipment and free maintenance and they get their product in another store with “free” advertising to boot.
as far as something “quick for the summer” is concerned, i have found that almost anything short of a reach-in case (for bverages) or a novelty freezer for pre-wrapped ice creams involves work. there’s a reason most successful ice cream and smoothie shops don’t also sell sandwiches and salads.
soft-serve machines are great and a good, pre-owned one can be had for about 3k. counter models are nice (which i had) but the cooling fan can be very loud; they also can generate a lot of heat and make behind the counter areas very warm. there’s no doubt that they are very profitable, especially when compared to hard ice cream, but if you aren’t selling the heck out of it i don’t think they are worth the investment as the product does not keep well. they are also much more labor intensive than hard ice cream as most counties require that you sanitize them every day - this is a process that can get old quick for some. but if your soft-serve is a big hit and you move quite a bit of it, it’s great - especially for milkshakes.
you might want to take a look at rich’s bahama blasts (smoothies) program - a couple of years ago they were offering tremendous deals while they were trying to position this relatively new product; they would give you state of the art blenders free of charge (maintenance as well) and great pricing on their product.