Lexor: I'm so sorry for my lack of response - lately I am a little busy :(
Thank you all above for your comments!
saramakos: While technically correct, "a great number of years" sounds a little awkward, perhaps "many years"? That's purely personal opinion of course :)
Also, cordially doesn't seem like an issue to me, but I guess that also comes down to taste. It is moderately formal, but does seem to fit the context.
Lexor: "many years" sounds as a good replacement :)
"cordially" - phew :D
JudasIscariot: "In addition" needs a comma after addition :D
"With use of my own or the client's material I am able to create a variety of DTP" there's a comma missing between material and I.
" cordially invite you to familiarize yourself with small part of my portfolio which can be found on this page" should be an " a " (without quotes) between with and small :D.
Lexor: All your comments sound nice - thank you!