Well you certainly have some good tractors to choose from. To get to your question about horse power. Many of the smaller tractors today have a lot more horsepower for their weight then traditional Ag tractors did or still do.
It gives you higher speeds when going on a hard surface and more power at the pto. to run increasingly more complicated and power hungry attachments. Pulling an implement in soft ground though you run out of traction before you run out of power and you can spin yourself stuck faster.
For comparison look at some you mentioned.:
Massey 1742 ......hp. 41.6..... wt w/cab......... 4068 .....wt/ hp 98.
Kubota
L4060.... hp.. 42 ........wt /hydo/cab... 4134 .....wt/hp ...98
Koit1 DK45SE .... hp.. 45....... wt /no cab....... 4625... wt/hp. 103
JD 4044M .... hp. 43.1..... wt hydro/no cab 3770.. wt/hp 87.4
JD 5045E........... hp. 45 . wt no cab 5070 .. wt.hp 113
Now that might not seem like much difference but if you have a baler on behind you pushing you down hill having another thousand ponds on your side helps keep you from being pushed around.
Just something to consider as you shop.