What would be considered "enough traction" before going for a seed round?

We have developed a software product (application platform) on which we have built five solutions for a particular vertical, in partnership with a paying customer. The intention is to deploy these into the cloud as SaaS applications with our platform underneath. Eventually, we will be moving into other verticals but have chosen to start with one in which all customers are "me too" - they have the same processes, same organisational structure, same drivers and challenges, etc. I have recently started reaching out to potential customers in this industry (we have worked within this industry in a consulting practice for many years and therefore have a lot of contacts) and the feedback has been very positive to the point that a number of these customers have indicated they would like to buy, and are at various stages of the buying cycle. This is an enterprise product where one solution runs around $10-20K per year (most of these customers want multiple solutions), so we are not talking a $19.95/month online application. In order for us to support new customers, we are going to need to be able to focus on this company full time, as opposed to the bootstrapping, off the side of our desk model we have been working under so far. In speaking with various investors, the feedback has been that we should get as much traction as possible before looking for a seed round. How much traction is enough? With two paying customers in three-year contracts, and a full working solution, positive competitive research and positioning, good response from a handful of customers, what more should we be gathering in terms of evidence before we look for funding? We may run out of options soon if these next two customers sign up.