I think when we're speaking about "limited resources" we actually mean 3 things: time, people and money.
So I will try to cover all of them:
The best time to start building your brand is the moment you've got your idea for your business. You must build brand awareness while you're building your product and you should communicate the progress with your audience.
That way when you actually launch you will already have a community that you can work with.
Spend as much time as you can and start doing it as early as possible. Simply because your products/services are not going to sell by itselves.
Most of the startups I know hire too many engineers and not enough marketers. It's very rare that you will succeed with only 1 person dealing with SEO, content development, community building, PR, etc. Have at least 2-3 people in your marketing team or consider oursourcing some of the work.
I have an inbound marketing agency myself and can offer you a fair price for an exceptional service and results.
Your marketing budget should be around 10-15% of your funding/revenue. I suggest you invest more into inbound marketing activities to see a bigger return of investment.
It is hard to tell where to put your money exactly because I don't know specifics about your business. If you need a marketing advice, don't hesitate to call me and I will try to help you in the best way I can.
I'm giving you a VIP key, so you can call me for free