Is this cost effective? Ie: Do you make money back from spending it on SEO..
If they can get you 1000's of directed organic visitors daily or weekly and this is a good thing for your business then yes, go buy it.
The problem is you don't know if you will make that back until you have tried it. Only option is PPC, for which you would need a site.
We are in a very, very competitive market and the total on-line platform will be costing over £40k to design, set-up and implement. And on top of that the on-going costs such as SEO.
At that sort of price tag, I would look at getting a permanent employee as one possible option. You will, of course, need someone with a broad skill-set.
One of the problems with agencies is they have high over-heads, lights, internet, phones, receptionists - and as their client you have to cover all of it, so prices are highly inflated over the actual work you want doing. The breakdown could be something like:
£150 - mean wages for developers, designers, SEOs etc.
£150 - utilities, rent, rates, technical costs, commodities, admin staff
£50 - management's cut
Also a lot of agencies are ruthless. Profit margins in web development are usually very low, SEO is variable. As such, it is more profitable short-term to not provide clients (who generally don't know much or anything about web dev.) much for their money. Having been under the bonnet in this game I have witnessed people being taken for a ride - such as being charged £20K for a site that cost about £800 to build; £180 a month for shared hosting, which shouldn't cost more than that a year; unbelievably lopsided equity deals.... You only have to look through the archives here to see the abuse.
My experience is also that a lot of MDs don't know much or anything about web development. This naturally affects what they produce and they can't even tell if they are hiring talented people. Their passion is making money. Whereas, MDs who are web developers are often passionate about making people the best site they can in their budget.
Further, my experience is that many web developers are bored and under-motivated. They have no interest in their conveyor belt of projects and so just bosh them out. When I was working in this field, I was most productive when I was dedicated working on two sites, with which I had carte blanche across the board. I was creative, independent and motivated and they are still the best sites I have worked on.
Further still, selling online is a hard nut to crack. The market is saturated with people with £300-3000 online stores selling stock which they often don't have. People see it as an easy money maker, but it's not. Even with SEO, most of these sites will only convert 0.2-1% of visitors; averaging from a few sales a month to say 10 a day. The main issue with selling online is lack of awareness of your site. People don't like buying from sites they have never heard of.
When you have an agency, your needs for their skills are likely going to be translated into - "How can we make the most money from this?" Whereas if you have your own developer, their £ is covered, and your needs should be translated solely into how they can fulfil them.
The only eCom. sites I am aware of that generate enough money to run a dedicated business and provide a living to the owners are ones where the owner(s) are developer OR they have relevant web skills in-house.