Rocket Espresso Milano - Boxer Timer Evo (2-Group)

I have seen specialty shops running Rocket Boxers or similar. ( 2 head group 2, about 8K ) But yes, the cafe where I work does the cold brew method and then mixes in powders and syrups to make frappes, slushies, ( pre-mixed fruit concentrates )and so on. They have a small commercial fridge, two work tables, a pile of syrups and mixes, and one Bunn coffee machine that soldiers on for well over a decade now. Typically you'd have two, but we make several containers of cold brew a night to stock ahead of the rush. Easy $1K for the machines, then another 1-2K for the fridge. The rest is workspace and supplies. With this and proper ratios ( and better syrups ), we consistently blow the Starbucks nearby out of the water on price and taste. Espresso isn't technically required for most mixed drinks. The other place I work at does do espresso, but that's it's own 15-20K adventure. That said, once it's all set up, you're making good money and paid it back in a month or two.

Rocket makes some great HX machines.

I’ve had Rocket HX machines for the past 14 years and never had a problem making milk-based drinks or entertaining. I think double boilers are overkill but to each their own. As commented below, a $20 smart plug solves your preheating problems. Who says the water reservoir doesn’t come out? Is that a thing? Mine does. Plumbing in would be preferred but my space isn’t big enough in the kitchen so my machine is off to the side on a coffee cart. Happy coffeeing.

would definitly look into young used machines, so many people get into this hobby and then stop because they think too much work involved. I'd look into a Rocket, ECM, Profitec, lelit etc... Minimum Heat exchanger boiler. (I have a Rocket with HX boiler, make only 4/5 cappucino's a day without any problem) and a decent grinder.

Rocket has finally shipped their fast prosumer models. Have a look. They stuffed a pid-controlled cassette into the brew head. Unless your side hustle is an espresso stand, HX will work.