Certainly sounds like you know your market niche and how to work it. I say go for it.
However, I would offer the following words of advice:
You say you are a skilled engineer and I don't doubt that you could, in time, service machines yourself and possibly save money. However, in the beginning, just get a machine either new or refurbished from a reputable supplier that will service and maintain it. Your time and energy will be needed setting up the business, doing the actual printing work, dealing with clients, and probably servicing your finishing equipment that you are buying used.
A used machine without a service contract will require maintenance, it will involve lots of your time, frustration, research, head scratching, downtime and sub-quality product in the process. Are you going into business to print or to service machines? Perhaps a compromise might be to get your main colour machine on a service contract, and then perhaps used B/W and a secondary colour machine off contract for you to tinker with and learn how to service. You need a main colour press you can rely on, otherwise you will miss deadlines and disappoint clients with shitty product. Reputation is everything, right?
You've never done this before and are talking about getting a used Indigo and servicing yourself. An Indigo is a very high volume, complex, expensive piece of equipment. It's not a piece of equipment that someone would start up with, when they have no experience and no service contract. You're making this way too difficult for yourself.
Get multiple toner based machines which are simpler to work with and so that you have redundancy. It's all good and well having one machine which is super fast and productive, but then when (not if) that machine is completely offline or having quality problems, you then have all your eggs in one basket and can't produce any product. On an old machine you may have little annoying problems which are not easy to solve despite repeat efforts and buying lots of expensive replacement parts.
If you're going to get multiple presses then think about getting ones from the same series from the same manufacturer, so that you have a consistent output across the machines, so that you really have redundancy across the machines.
this guy's youtube channel is good. "Just a Printer"
https://www.youtube.com/channel/UCSuSPbvmwLZv9CMdeSsWkFA/featured
he has several KM machines - one on a service contract, and one which he bought used for peanuts. He made a video where he explained the process he went through to calculate the offer he would make for the used machine which he was effectively buying blind - he bought it knowing it had problems but not knowing whether it would be economically repairable, or if it would just be for spares. He was lucky in that it was simple to repair and get running again, but it could just as easily have been a pile of scrap.