Caveat: Links to 3rd-party sites may be obsolete.
Use official WordPress and WooCommerce sources whenever possible.

SSS B2B Toolkit

SSS Payment Gateways

SSS Price Quotes

Recommended 3rd Party Plugins

WooCommerce Order Status Manager

This plugin from SkyVerge, while not free ($49/year), provides extensive management for custom (and core) WooCommerce Order Statuses, and also allows for custom emails. The SSS Price Quotes plugin makes extensive use of the features available with this plugin, and includes some customized email templates that work with it.

WooCommerce PDF Invoices, Packing Slips, Delivery Notes & Shipping Labels (Pro)

This plugin from WebToffee, while not free ($69/year), generates various highly customizable documents for your orders. The SSS Price Quotes plugin supports use of the ‘proforma invoice’ available with this plugin.

Flexible Checkout Fields

This plugin from WP Desk is free (but consider the Pro version), provides customizable fields for order address (billing and shipping), and general order fields. The SSS Price Quotes plugin supports ‘order’ fields ‘order_po_number’ and ‘order_job_number’ on the form_pay (Place Order) page to place them on the same line via the ‘sss_form_pay_fields’ filter at a low-priority, so it is easy for your developer to override and/or extend the displays. It also supports updating these fields, and the core ‘order_comments’ field, by hooking the ‘woocommerce_before_pay_action’ action.

Sucuri Security – Auditing, Malware Scanner and Hardening

Every site needs all the security it can get. This free plugin is from what I consider the top website security company. I would also recommend that you signup for their Basic Platform ($199/year) – they know what they’re doing. NOTE: If you are a GoDaddy customer (I also recommend them), the GoDaddy Security Plans are provided by Sucuri.

Full disclosure: I used to work there — and still recommend them!