Should I go for SEO or PPC?

Firstly SEO and PPC are completely different. You should definitely do SEO, that answer isn't related to the question of should you do PPC. So with the "yes do SEO" out of the way, PPC doesn't have to be a guess.

The best way to start with PPC is slowly. Make sure you set up enough Google Analytics so you can track who hit your site from PPC and also who converts from PPC (convert for you might be sending an email to get into contact). Once you know your conversion rate (ie. for every 50 visits from PPC you get 1 email), you can then work out the cost to get an email from your website via PPC advertising. Now you just need to find out your conversion rate from emails received to projects initiated which will probably be tracked manually. Once you have this, now you know how much it costs with PPC to get a project. With this information you can make an educated decision on if its worth it (eg. if it costs you $1500 PPC to get one project started, it's probably not a good method of advertising).

Obviously you don't have all the numbers yet, so get tracking going on your site first (through to conversion/goals) and then start slowly with PPC to get some information to measure.

PPC doesn't have to be guesswork, measure and calculate.

Optimizing your web site for search is all about trying to become highly discoverable to your target audience. And you should definitely do that, because you want your cost of acquiring leads and opportunities to trend town as low as possible. And yes, as you say, it tends to produce returns over a longer time period.

PPC (and CPM) advertising creates more control: you boost visitors that you need to convert into customers. And the way the economics work, the advertising platform tends to eat a major chunk of the long-term profit margin of a well-tuned site. (Or, to put that another way, during your early, tuning phase it's somewhere between possible and inevitable that it's going to feel like you're pouring money into a deep pit.) So use paid advertising to build experience and a contact base, to refine your model so you become one of those efficient operators who can make money, and to generate goodwill and customer stories that are your best source of sustained and profitable growth.

Budget-wise, you'll find a wide range of advice. In general, I would say that you should make your first financial commitments under each heading affordable, and aim to maximize learning. By flexing what's not working and scaling up on what does, you can build confidence that the money you're burning is fuelling growth and not just heating the atmosphere.

SEO is definitely a must for all companies who are willing to sell products or services online.

You shouldn't consider PPC as spent money, instead you have to calculate ROI and you have to continuously optimize your campaing on the basis of conversion rate.

1. First: calculate how much money you earn on a regular order (for example in your case you charge 3000 dollar for an app, from this you must have at least 600 dollar as pure profit).
2. Create individual LP-s for your ads, and configure conversion tracking. This way you will be able to calculate your returns on each clicks, and optimize your campaing on the long run!
3. Go for ROI not for Clicks! - Do not make to good ad text, only attract those who are interested in your offer. For example if you know that you are willing to sell app development services for 3000 dollars clearly state it in your ad. This ways those who are looking for a cheaper service provider won't click your ad - so it cost you no money.
4. I think in your case conversion rate should be at least 10%, and I would try to optimize the LP and ad texts to reach this rate - this way you can minimize your costs, and only get to those who are really interested!