am i out of my depth? SEO Please help.

right.. long story short(er)..

i have been doing some local SEO for some SME's around my local area, and have generally been quite successful. i have a fairly good 'gist' on how SEO works, and with local SEO its seems quite straight forward.. good keyword research, a targeted domain name, good relevant backlinks on relevant and social networking sites and minimal competition... simples. some of neat little tricks i have picked up from alot of you guys have worked a treat aswell!
(thanks

i have been trying to build up a client portfolio for local SEO in order to get myself more business and eventually make an income through it.. recently i have been approached by a company i tried to get a dropship account through (unsuccessfully), they are are national pond and aquatic supplier and they want me to do the SEO for a spin off sister site for them. GREAT NEWS!! i cannot turn down the opportunity and really need some advice if possible.
i feel i may have jumped in at the deep end a little, (which is the only way to get ahead these days so i have to go for it.)
my main problem being that as this is a sister site to there main site, and they are selling the same products to the same market and no doubt will be using the same keywords.. im worried i will not be able to compete with the competition. especially because i know one of the main sites i am competing againts is their main site (which i know is maintained by a large professional SEO company)
i am willing to put all the hard work (and more) in, as this could be a big break for me.

previously with other business i have worked with (literally only a few) it has been quite easy.. for instance, for a local photography agency, i did abit of research and found that the best keyword was "(area) photographers".. so to me, the only domain needed was "area-photographers.co.uk". with a few relevant backlinks and little competition for this keyword, it worked very well and shot it straight to number one under the chosen keyword..
this will not work for this new project as they have a domain and site already.
also, with photography being a pretty straight forward service, this made it all abit easier, the main and only keyword i felt i should optimise for was an easy choice. infact just the one keyword was enough to get traffic booming within less than 2 months!
but with a large pond/aquatic equipment site, there are alot of products, webpages and ALOT of keywords to optimise for..
is the principal the same with local and national SEO?
is it really just a simple case of relevant backlinks to your site and good keywords? i understand that with the new penguin update, techniques like article spinning are a no go now, and i should focus on social networks and relevant backlinks.. i have some cool little tricks from posts on here, but i was using them for sites/keywords that had hardly any competition..

What exactly do i need to do?! i know that seems like a very nieve question, but i have only really optimized a site for 1-3 keywords as it was all very niche targeted services. "basingstoke photographers" "aldermaston skip company" etc etc etc.

how exactly do i optimize for more than one keyword? again, nieve question, but is it simply a case of working your keywords into your content, and then having a good backlink network to the site? what about anchor text and stuff like that.. say im 'backlinking' and i leave a link on a relavant forum or similar.. do i anchor text a keyword? if so, which one? do i leave multiple links with different anchor text? this cant look good surely? also if i have backlinks to site, will that push my page rank up for all keywords, or does each backlink i produce have to correlate to a specific keyword? obviously the site's content will be full of keywords anyway "pond pumps" "aquatic lighting" "fish tank gravel" etc etc etc. so by generically backlinking with no anchor text will push the rank up for all the keywords within the content anyway wont it??

ANY general SEO help for a site like this is very much appreciated. as i said this is a massive transition for me, going from local to national. and all help is very much appreciated.
ps i will have admin access to the site, so meta tags and on page SEO will be easy.

i apologise for the essay, i do like a good rant.
thanks again for any help in advance.