Questions for webdesigners/programmers/database admins- Help!

[Cuppycake]

Okay, don't flame me...here goes.

I have had an idea for a website for a long time, a web community for horse sales and events. There are tons and tons of websites where you can pay 15, 20, 30 bucks to host pictures and advertisements for your horses for sale, boarding stables, events/clinics, etc. (See www.equine.com, or www.horsesmidwest.com) I would like to own/admin a website just like this.

Basically I'll throw my ideas for the website so that someone can help me by telling me what kind of startup cost this would entail, the level of difficulty, and if it is even possible.

*I would like the website to be possibly portal/php based, in that I would like a single username/login for the classified system and the forums.

*I would like a messaging or PM system that is universal across the whole website.

*I would like everything automated as far as classified payment goes...I want the user to be able to go to the website, purchase an ad, pick which package they'd like (how many pictures, etc.) and pay for the ad...all with the ad showing up after payment is received automatically without me needing to approve.

*My ideal image for the website is something like stratics or allakhazam. You go to www.allakhazam.com and from there you pick which game you want (EQ, WoW, DAoC, etc.) For my site I would like the user to be able to pick which genre of riding style they do and have the website change to only list the horses that the owners have specified would be good potential for those discliplines. For example, you go to www.myhorsesite.com (or whatever) and then you click on "Show Jumping." The website changes to www.showjumping.com (or whatever) which looks very similar but different in color but ONLY displays show jumping horses for sale, user submitted show jumping events, etc. Or it could be something like they could pick their location from a dropdown menu and only show the horses/events from their specific region.

*I would like the website to be smart...where it watches which ads you are viewing frequently and pulls similar ads up to the top.

*I want the website to look GOOD. www.horsesmidwest.com is a very popular local horse community, but the website and its use of frames is horrible. Its very 'not current' and I'd like to be competition for the website.
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So my questions are:

-How difficult would something like this be? How much are we looking for startup costs (excluding advertising) for something like this? Would we be looking for a website designer or a programmer or database admin or what?

Thanks in advance and hopefully you'll understand that I'm a horse person, so although the subject matter may not sound appealing..hopefully i'll still get some good responses. Thanks!