Welcome to the wonderful world of D4 ownership.
I'll try and answer your questions in order:
1. Replacement front bumpers are not available seperately. You do have to buy the front end pack.
2. IMHO opinion, you'd be best to hang on for a brushless set-up if you can wait long enough. It would be better than buying a brushed set-up and then have to fork out for the brushless kit at a later date.
Nosram do some very good mid spec brushless stuff like the 'Evil' it will run upto an 8.5T brushless motor, but nothing lower. It really depends on how fast you are wanting to go.
http://www.mirageracing.com/piw.php?partNo=NR90400
Cells: 3300mAh or above will be plenty for the spec of racing your looking at. Brushless motors are so much more effiecient than their brushed counterparts so expect to be able to last with ease.
Radio:
There are some good cheap starter sets out their. Try and aim for a servo with over 6kg of Torque if your budget will handle it. Speed: 0.17secs (0-60) is plenty.
I think £200 maybe a little on the low side unless you are going ebay / club night shopping.