Software recommendation for 1 person small business.

Not sure if this is the right place to post this question but I run my own small business and here is how my work flow is currently:

1. I get a cold call from a new client
2. They give me their contact info, and details
3. they describe the issue they are having
4. I quote them an initial price, and they either go through with it or not.
5. If they go through with it, I use folders on my computer to keep all the related files in one place.
6. I bill them through a variety of means, and track it in the word document associated with the customer.

I want a software that can do the following:

1. (Hopefully) one time cost… no recurring subscription.
2. Has a place to enter customer information, and any notes or documents associated with them
3. Can link to various billing software to track invoices.
4. Calendar integration, so I can see what my schedule is like.

Any Ideas? Or recommendations for other subs I should check out?

Thanks!