Installing WordPress & Using for Multi-site building

Hi there everyone..
I’m a new user to wordpress but have come to realise the potentials of installing it vs the online version. However, I have a couple of questions that I hope I can clarify through you, since they’ve confused me so far and have not allowed me to proceed with installing it yet.
1). As a new designer, I want to install WP and be able to use it time after time building several sites. From what I gather, I have to purchase a webhosting acc. and use the “one-click” install of WP. However, once I’m done with the particular project and hand it to the client, I will no longer be able to use the installed version of WP, am I right in this ?
2). If above is right, what is the best option for me? Have I completely misunderstood the usage of the installed WP?

Please someone, can you give me a guideline of how this works? I previously built my own business website, (irrelevant to web designing), using a different platform. Having checked with my Web Hosting provider, (GoDaddy), I need to pay them extra $$ to install WP, so I’m thinking of starting with a different web hosting company for client purposes.

Thank you sooo much for you time in trying to help me on this…