Hey man, welcome to the club! I will try to answer a few of the questions you've asked.
1 K&N Apollo intake - there are several topics in the MD/UD area of the forum that might be able to provide you some guidance on the install. Probably the "hardest" part is removing the stock intake and the air resonator box. Check out this thread
here for some pics of how to install (see all pages). The easiest way to get the box out is to either remove the bumper, or remove the driver side front wheel and then you can access it from behind the wheel. To put the new one in it isn't that hard, as it's really just a few hose clamps.
2. Suspension - There are various routes you can go. The cheapest would be to just put lowering springs on (say between 200-300 plus install) but you will wear out your shocks faster as there will be more pressure from sitting down low. A good full coilover set will run 1,500 - 2,000 plus install. You will be able to adjust your ride height much more with this and is generally the "best" suspension available. Your ride will be much more stiffer. If you're going to shell out this type of money, stickier tires are almost a must or else you aren't going to truly be able to enjoy your new setup. Also strut bars to help reduce body roll will complete the job here!
3. For the interior, what are your thoughts on what you'd like to do? Are you looking for a stereo, custom leather interior, painting or applying film to the dash, lighting, etc? For the cloth on your doors, maybe your best option would be to have custom vinyl installed if you wanted them colored. I have no idea what price would be on that though, but you would certainly have plenty of color options to choose from.
Feel free to check around the club as there are lots of different areas. We have the main forum, but then we also have a DIY section, Reviews, Wiki, Chatroom, Blogs, Member Showcases, Garages, Car of the month (COTM), and an arcade to name a few. The arcade is only available for those that go "VIP" which is basically a way to help support the club and help Mitch (ElantraGT) cover the costs of running this club. There are also lots of other perks of going VIP (ability to upload pics stright from your computer, add pics to your garage, ability to change your display name (which I've used several times as I originally started here as Alex2004GT), and much more. You can read some more about the community
here.. This (unlike many) is an independent forum, but that is why I like it so much!