I know of course you can DIY it, but i mean is there any seo people who take an ad-hoc project and aim to get it ranking well for the foreseeable future?

Categorically no. SEO experts can optimise a site to the best of their ability given current SEO best practice but if the search engine makes a change to their algorithm the site could disappear. You could of course get lucky but in my experience you have to monitor sites and constantly tweak them if you want them to stay up there.

As BDW says, you can't get 'years' of guaranteed performance from a single project, but then it is entirely possible to get a fantastic SEO job done as a project that does not involve ongoing monthly fees. If this job also involves some skills transfer and some education, the client may well be able to defend the good performance for a fair while.

Steve

Of course you can do a one-off and be successful, but how would you continue to seek link developement and build new opportuinities.

In addition, any search engine can change its webmaster guidelines, algorithm, and strategy at any time.

Importantly, do you think your competitors might be sitting around doing nothing to get ahead of you?