Contracts and deposits for development.

I've done a variety of freelance projects for both development and design at this point and I generally reuse a template contract if the company or person does not provide their own. It's fairly basic - outlining your responsibilities and explicitly where they end (let's say if they request a variety of non-bug related updates, further compensation may be necessary). You can find some good ones you can tailor to your situation online. Most of the projects I've worked on do not come with much, if any, workable UI design prototypes so, like an architect, I'll sketch out preliminary plans for what it's going to look like and comment on reasonably how it's going to work. If they have agreed to work specifically with me, I generally do not charge for this initial few hours of work as I would like them to be confident in their choice with me. I've found it always to be better to have a generous relationship with contracts as if they have a good experience, I've seen a good amount of repeat work.

After that, and after signing the appropriate documents, I generally request a small deposit (more around the 10% range) as a way of proving they are serious. At this point though, it depends on how you are getting paid. If you're straight hourly, then I don't charge an upfront as they are contractually obligated to pay me for the time I put in. If it's by milestone or full project, then I do. Now I have no idea if this is the way you are supposed to do this but it's worked out swimmingly for me so far.

The hour ballpark is something very specific to yourself and to what you (and the client) want to outline. For example, a basic one sans animations, accessibility and language inclusions, extensive testing, etc. may take X hours but add on things, even small things, can bump up the hours. It also depends on what they expect you to use a a backend as learning or hooking into new backends is a chore. You're a mod so I assume you've made complete apps before even if this is your first client. You know what you're stellar at and what takes a bit longer. Also - tip (at least for me, maybe you already do this!) - download a snippets program (I use SnippetsLab) and start filling it with useful things you find on SO and you write yourself. It saves a tremendous amount of time. TBH for your first client, worry less about the money and time (within reason, of course!) and worry more about running through this first experience. You learn a lot just by having a client and working out that relationship. that you can bring to your subsequent clients.