Don't reinvent the wheel. Igloosoftware is a great system it's saas too.
I agree if it's for learning go ahead and have fun. For company and production find a SaaS company.

Aneil

If I am not mistaken your customer needs to add a SPF record in their DNS as such.
IP address/system name your emails are coming from? ie: smtp.manhattanpizza.com

"v=spf1 a:smtp.manhattanpizza.com ~all"

What this does is tells receiving email servers that your customers domain approves emails being sent from smtp.manhattanpizza.com

Hope this helps.
Aneil

Just throwing it out there that I own www.simplyappointments.com

My question to you is how large is your city? Sometimes people try and promote too large when customers will no do the travelling to get to you.

Google/Facebook allow you to specify your target market, age, location, and much more.

Word of mouth in your business I think is king though. Make sure your current customers are super happy and let them know you want more customers.

And yes, being able to book appointments online is pretty damn important. for me anyways.

Hope this helps you some.
Aneil

Why are you hiring as an employee? Why not hire as a contractor and let them worry about taxes, etc..

I very much agree with Bakul Kaushik, you will save time and money.

Others talked about security, I use Cloudflare. $20/mth gives you a CDN and Security.

Make sure you have people who can support the underlying system for Wordpress... ie; Linux, Sendmail/Postfix, MySQL

have fun and good luck in your venture.
Aneil

What technologies do you know? or are you outsourcing the development?
If you are worried about software costs for Microsoft there is a Startup Bizspark program where software is free.

Using Opensource is also great, there are so many options. Don't think you could go wrong with LAMP and server hosting end up being cost effective as well.

PHP and MySQL are powerful.

Determine how much the custom modification is going to cost you in time and cost. Is this a fast track project? so something you have already considered doing but has less of a priority on your current roadmap.
If this is something that is on your roadmap you may what to not charge as much for the fast track. Of course you need to determine this.
If one "lighthouse" company wants your system with some mods then why wouldn't "lighthouse2"? Expanding your market is a good thing.
At the same time how much is this project/mod going to affect your focus on your system and roadmap? Can you afford to delay your roadmap?

As for branching, you need to figure out a way to add this Mod as a module allowing you to easily integrate. You should be building your system in a modular manner anyways. Turn off/off features easily based on customer wants/needs.

Figure out a way to do the mod for the light house customer. :)

When you go with Cloudflare you will have the ability to use or not use the CDN. The great thing here is you can see if your site has problems quickly. Make sure your QA team goes through your site to ensure you find any issues.

It is definitely a risk to integrate with any other company. WHY? If that company has a problem your customers will not know or CARE. What your customers will know is your SaaS app was not working.
Now. if you are starting up you have to weigh the time/cost to integrate and build yourself. Risks? Pros/Cons?
If the integration can be done quickly and for low cost you can plan to build your own later and you will also have data telling you if this integration is wanted/needed.

Hope it helps some.

Do you have competitors? How much are they charging, charge something close to them. Don't undercut majorly or you're indicating your services can be cheap.

Have a look at out pricing page. Free model, al e'cart , free trial on paid plans. add-on's to paid plans. http://www.simplyappointments.com