First and last rule of choosing a system in this cloud era, never choose a client-server system.
Here are some of the business system recommendations for a startup:
ERP system: All startups start with Quick Books since it is cheap and very robust. There will come a time when you start hitting $1M in revenue and look forward to scaling fast. This is the time you bring in an experienced finance hire and a good system. I would recommend two in particular, Financial force and Netsuite. FinancialForce is built on Salesforce platform and is very easy to use and at least 8-10 times cheaper than Netsuite. However, financialforce is still a work-in-progress. Netsuite is the best of the breed for startups. Don't yet think about SAP or Workday, they will cost you an arm and a leg.
Planning Tool: Excel has many disadvantages but it is the easiest to use. However, you will need to start thinking about a tool as you rapidly grow. Adaptive Planning and Anaplan are your best bets.
SalesForce: I guess there is no competition here.
Expense Management: There are plenty to choose from. I would recommend Concur or Expensify.
Invoice Payment system: Go with Concur or bill.com. Both have extensive routing and easy payment options. Concur has an added advantage of being integrated with Concur Expense management.
Applicant Tracking System: You can choose from Jobvite or Greenhouse. Get it early so you can have a headstart on the talent search.
Stock compensation Management: Your external legal team is already using either Solium or eShares to manage your cap table. Bringing it under your umbrella is very easy so you can do accounting stuff and get cap table as and when needed.
Xactly: There is not much competition here too. Xactly is the best Commission management tool and it will keep your sales folks up to-date and integrates well with Salesforce
GSuite : It will get you a host of collaboration tools like Gmail, Calendar, GoogleDocs, GoogleSheets and GoogleSlides without the hassle of Microsoft suite.
Business Intelligence(BI) System: BI systems become important as there is so much data and you need a single source of truth. Invest in either Domo, Tableau or Zoho.