That's good that you're careful with your budget, as I know many startups collapsed because of great costs in the beginning. But without more details in your response it's hard for me to give you recommendations, I can give you only general information about business software. For every company main things will be:
- accounting software which helps you to control and balance your finances, also provides an insight into your customers base (such programs as FreshBooks, Xero, Zoho)
- documentation software which is one of the most important ones because it'll provide all information about your company including documents, presentations etc. (basic Microsoft's Office package, OpenOffice, GoogleDocs and so on)
- inventory management software, it's a basis for high level of organization and control for all your business functions and ut makes your general work much effective (ABC Inventory, Magento
https://marketplace.magento.com/amasty-extension-customer-activity-log.html
, InvenTree)
- programs for developed e-mail network, fast and convinient communication is very important too, and it's the easiest in case of software (Gmail service for example)
Also many such programs can be found for free but they rather often have limits for functions as I know.