Congratulations!. As far as what to get next- my vote would be1- Brooks saddle- the more you ride the more you will need it.
A heart rate monitor is really only usefull (in my opinion for those who beleive they are training competitively.
For bike computer get the cheapest one you can get. Most of these electronic gadgets (HRM included) is that they can be programmed to give you all sorts of information- most of which is not needed and might not be useful.
I have 3 brooks saddles, one on each bike, I have an old cateye that was a gift years ago and now only records total mileage on only one bike(which is all I ever wanted) all other functions have died. I have a heart rate monitor that both my wife and I also used for about a year each and now sits in a drwer with a dead battery. I found that obsessing over my heartrate,average speed, cadence, etc all interferred with my enjoyment of riding. Obviously others may feel differntly. Good luck