catacaustic on "Multible Websites on wordpress"

Can this be done using only one wordpress installation

Answered by esmi above. :)

will this effect the SEO of the sites

Most likely not. The only possible "issue" is that the sites will all have the same IP address, which is not a big consideration at all these days seeing as how the vast majority of sites are on shared hosting so sharing IP addresses is standard and expected these days.

cause complications to all if one encounters a problem

Yes, and no. :) It dependds on the "complication". If there's a bad update to the WordPress core or a network-activated plugin then yes, all of the sites will be broken until it's fixed. If it's a local plugin or theme them it's most likely that only the site(s) that use that theme/pluign will be affected. Of course things like database problems will affect all sites at the same time also.

Installing everything as a separate installation will take time, i feel it will be more secure but will this bulk up the files on the server?

Of course, because you'll have more files on the server. To me that's no reaason to not use separate installations.

If I was in your situation I'd definately use seperate installations, and I'd also try to separate these into their own hosting accounts as well (where ever possible). That way if one site goes down for whatever rason, nothing else is affected. Every site is different, and there's no need to share data or users, so having everything all set up in one central area for the sake of saving a bit of time in administration isn't a good trade off for the security of keeping everything seperated out.