The 400D is still a very good camera and will be fine, for the foreseeable future anyway! Don't worry about the 10MP sensor. I still use the 30D - which is a very slightly upgraded 20D - dating from 2006 and it's 8.2MP. I don't have any complaints.
The 18-55mm kit lens is decent, fairly versatile, and there's no particular reason to look for anything else right now. It's a useful range for many purposes so use it, learn what it can and can't do, and what is important to you. This will make it much easier to decide what you want/need next.
I have mixed feelings about the 50mm f1.8 lenses. Primes (fixed focal length lenses) are generally sharper than inexpensive zooms, and they do offer a fast aperture. This is useful in poor light, and also lets you isolate the subject using the shallow depth of field, but I find 50mm too long for a walk about lens, and too short to be a useful telephoto, on a crop body. It was popular when many of us used 35mm film, and serves much the same purpose on a full frame DSLR, but this changes on a camera like the 400D. You can experiment with your 18-55mm set at 50mm if you want to. See if you like 50mm and then decide if you want a prime?
I'd ditch the UV filter. Digital cameras don't need them for UV protection, which is a hangover from the film days. Some people use them for physical protection, but many of us have our doubts about this. It's controversial, because budget filters can cause flare and softening of the image under some conditions, and there are a lot of 'debates' on the forums about it. Just do a search if you want to find out more about this.
One thing I'd strongly recommend is a lens hood. You don't need an OEM Canon one - they're pricey - and a generic hood is fine. It prevents light hitting the front of the lens at an angle, reducing flare and increasing contrast. They also offer some physical protection. I've been using them for the best part of 50 years, I don't use UV filters unless conditions are extreme (blowing sand/dust/spume), and I haven't damaged a front element yet...
Have fun
