First, let's focus on your actual problem: you've lost development productivity.
So your first question is, can you pull back some quality time, so you can get back on track with the roadmap?
It sounds to me that your issue is switching. Prospective customers want your time: that's excellent news for your start-up! Existing customers want your time: that's key learning for you! But the hours that are left aren't productive, because you're feeling out of control and losing focus.
Your immediate priority isn't to hire, it's to get on top of those two tasks yourself.
Step one: design
Come up with a "template day" that combines pre-sales service, after-sales support and development. Maybe that's three blocks of time, each labelled with a single activity. Maybe there's some logic: if there are more than x outstanding pre-sales calls, this time is for sales, and so on. You already have a good idea of the typical balance of your time, so use that insight to take control.
Step two: measure
Start each day with a sheet of paper (real or virtual, whichever works for you), set out with the time blocks, and with three columns. The first column is for how much work you have outstanding or allocated to the time. The second column is how much you get done (so for customer support, for instance, I would just use five bar gate counting for the calls you close off). The third column is to track 'intrusions' - stuff you did that didn't in theory belong.
Step three: learn and improve
Go through the day sheets and see how it's going. Is the work stacking up? What's the pinch point? Is there a different shape to particular days of the week? How often does it all go wrong, and why?
This may lead you to tweak the shape of the day, or to vary the days of the week (for instance, maybe to be productive on development you need to carve out half days, but you can't afford that every day of the week).
You'll most likely find that you want to refine the recording. Don't go overboard, but if one category of work is breaking down into two types of activity, you can capture that. You've over-refined if you can't keep real time records simply and naturally.
Step four: hire right into your need
Maybe all you needed was some process. But chances are you do need some extra resource. The work you've done gives you the insight you need to explore options. For instance, in pre-sales work, maybe you need to have someone take that on in its entirety, or maybe you just need someone to take on the admin. Or the daily workload may be so varied that you need someone who's more of a generalist.
Because by now you've structured the work, you have a pretty clear idea of how many hours of help you need and some realistic metrics: you can describe the job(s) you're looking to fill, train someone up to complement or replace you, and see how it's working out objectively.
Remuneration
At this stage, I'd say keep it very simple, and stick to the same lightweight tracking. . Chances are, your first week or two is going to be less productive, so don't make hasty decisions, but do measure progress. At a minimum, you want all the pre- and post-sales work to be covered, but with extra hours freed up for development.
Pay an hourly rate: you really don't need the distraction of dreaming up a bonus or commission system. Be generous: if in the new set-up sales start to grow, make sure your new assistant feels included in the success. Scaling simplicity is simple: you're adding hours.
Job title
If in doubt, your new hire is in "Customer service." (So are you, if it comes to it!) Some time in the future you may start to feel the need for specialist roles. But early stage, you want even specialists by experience to be people who are willing to do whatever's needed, just like you do. And you want someone who's going to fit in to what you've built, and improve it over time.
It's an exciting time for you, as well as frustrating. Be methodical, and you'll save yourself all the pain so many startups experience when they hire to solve a problem they aren't on top of, instead of hiring to extend capacity they have.
