Just because you're planning on operating a low cost operation it's foolish to think cheap. Instead, you have to think what is going to be cost effective and what will work for you and your budget.
If you've not considered Outsourcing then maybe that's a good solution as you can make use of what an existing support company has without having to spend a fortune on a system you might never get a decent return on. There are a few companies out there who have the capability to offer this as a service.
Systems you might like to consider which won't break the bank include
Air Support PPS, Air Data, Navtech NFP & a whole stack of others. Eastern Airways in HUY use Air Data so if you want cheap, copy them. I can put you in touch with their sales guy if you need it.
Have you also thought about contacting some existing Air Taxi ops and asking them what they use? They may not wish to answer that but if you don't ask you will never know.
If you want info i'm only too happy to help. Besides, i'm currently available for any freelance Ops, Crewing & Commercial Sales work and i'm only up the road in Amsterdam.