What type of roofing do you have exactly, clay tiles or cement? Because that changes the approach quite a bit.
Either way, I agree with people saying this isn't a job for your average electrician. A company specialized in solar energy is really the way to go - they know how to work with roofing without damaging it, how to seal the holes properly for waterproofing (that's where most leaks start later), and they're familiar with Lisbon's local regulations. An electrician can handle the internal wiring afterwards, but the structural installation needs people who do this regularly.
As for budget, it's around... well, it depends a lot on the size of the installation, the panel brand, and how many you're thinking of. Small to medium installations fall in a pretty wide range depending on the supplier. What I'd recommend is getting quotes from 2-3 companies in your area and comparing - they always offer a free initial assessment, and that's when they'll see if your roof needs structural reinforcement (something some of Lisbon's older roofs definitely do need) or if it's straightforward. That assessment will give you a realistic idea of the final cost.