Negotiator. Outsourcer. Startup builder. Fast-track startup developer. Business Partnership/ Co-Founder expert. I work with founders to follow the build quick, test & then measure methodology. This process allows you to quickly turn an idea into a business or shut it down. I have a background in finance & legal administration and working with co-founders to quickly bring an idea to market and then go back and optimize the business to scale.
I have very successfully built businesses on an almost entirely outsourced model.
One company I have has 6000 clients across Australia, New Zealand and the UK. We outsource the sales team on-shore and the product delivery & customer service off-shore.
Firstly its a decision based on wants and needs - "I like the idea of a particular business but don't want to deal with staff/customers/operations etc" - so understanding what you don't want do, helps you align what is worth outsourcing.
The key with outsourcing is use multiple providers for both the same functions and also the different functions.
If you only have one provider for a function you are always at risk of either rising prices or not lowering prices or being held to ransom in some way or another. When you have at least 2 providers you can always negotiate the best outcome using fear of loss and then the 'well our other provider is doing .... for us, can you please do the same?'
You are the secret source that holds all functions together if you are outsourcing say design, marketing & development - make sure you use three separate providers.
A) they won't want be able to replicate without you,
B) they won't push one department in a direction to make it better for another department.
Hit me up if you want to chat further. The model can rally really work for both growth, profit and lifestyle.
I'm successfully running a handful of different businesses all using Xero.com as the financial back end.
Two of my businesses (WebReception.com & Searchwords.com) are undertaking the exact tasks you need through the xero saas.
It covers the requirements you've noted above and much more. So consider using it to manage the whole of your business' finances.
It has a good API and a fair handful of apps that are built to work with it.
Zapier.com also offer a bunch of API scripts to communicate with xero.
Let me know if you need any more insight or real world examples.