My preference every time is direct response marketing when selling B2B.
Write things, with a compelling offer, send them to people.
Start by building a database of SEO providers - they all have websites, and create a mailing list from this.
Back up the direct mail campaign with awesome content marketing around the SEO space - not telling them how to do their core job (SEO) but other peripheral tools which are useful e.g. client handling, new business development, analytics to prove your success etc.
If they like what you write, they'll be more open to trying your app for their clients.
Happy to amplify around this area and scope out a strategy and detailed tactics for you to execute.

Here's a series of blog posts we wrote around a SaaS offering which we resell
http://creativeagencysecrets.com/category/feedblitz/

Should give you an idea of where to start even if you don't hire me to work with you (;-)

You can absolutely dominate with this (assuming your web app is excellent) by doing Facebook Ads. I recommend right hand ads that are targeted to people interested in various SEO keywords, groups and pages.

We do this for clients all the time and by far, Facebook is the absolute best (and most affordable) way to market products and services like this right now. The targeting, especially using the Power Editor, is ridiculously good.

I'd recommend testing 50 different ads and 2 methods:
1. Landing page that is on your Facebook page as a tab
2. Landing page on your actual website.

This works really well right now, even at lower ad budgets.

If you want to do the most damage with the least amount of money you should do most of the following methods your self or with your in-house team.

What I would do is to first setup a retargeting campaign in AdWords (remarketing as they call it.) Set a rule that if someone goes to the page and don't register or don't signup for a free trial, they will see x amount of banners for the y amount of following days. I usually show a banner 8 times a day for the following 7 days in order to do some branding, conversions but at the same time don't break my audience's #$%#.

Then having retargeting ready, i'd build 2 good infographics. The world of seo is full of great stats and SEOs are hungry for stats. Build up amazing stats through research and go promote the first. Where you might say? I've written an article about infographics promotion here: http://www.moneytized.com/howto-promote-infographics/

Remember, if the infographic is mediocre in design and/or in stats, it will only bring some seo value (if any) and nothing more.

with the rest of the money i would go and split it by half, taking half and test adwords search network and with the rest run a press release at prweb and some banner diplay in same niche blogs through buysellads, seeing which converts better.

To be able to give much better guidance i would have to know much much more than your budget, eg. ticket price, profit margin, your features etc.

When it comes to hiring a professional, since it's design driven, you'll need 2, one marketeer and a designer who will help with the banners for retargeting and buysellads and then with the infographic.
If you find a person who can say they can do both and they are amazing at both, run. :P

Depends on the product itself but i think you'll need more money in order to promote it.

If you need any clarifications or need to discuss more, you know where to find me :)
Cheers,
Angel