Well where do I start! I would absolutely reccoment coming to UCL. I'll try and answer your questions as best I can.
1.) Welcomey/homey feeling? Absolutely, even though UCL is not a collegiate uni, the campus is not very big and is concentrated in one main area. I see my friends walking around the campus and surrounding roads all the time. Always a friendly face to be seen.
2.) Making friends? I never played a sport before I came to uni but the best advice I was given and will give you to is to join a sports team. It is a great way to make a huge group of friends! You're also bound to make friends from your course and your student halls and any other socs. you join.
3.) LAHHNNDAAAN! An absolutely amazing city. UCL campus is pretty much in the centre which means walking distance to hundreds of amazing places - Covent carden, soho, camden, oxford street (which will lose it's novelty, trust me). So yes very walkable. Like any huge city it is very fast paced, most people will be in a rush, but you will get caught up in and it will be exciting. The tubes are fine as long as you aren't trying to go to Bank at 8 in the morning. Crime...you should be fine around UCL but ofcourse there is crime, jut make sure to keep hold of your bag etc, don't get paranoid though.
Lastly 4.) Reputation. WHAT YOU TALKING 'BOUT FOOOOO' everyone knows University College London, you honestly don't need to be worrying that UCL is not an employable uni. Pshh honestly :P
Hope this is enough?