im going to have a game soon with minas tirith with 300 pts and i'm thinking of having a bolt thrower so i can take down something nasty.

heres the list so far.

denethor
avenger bolt , 3 minas tirith crew members. siege veterans. swift reload.
minas warrior (shield)6
ranger (spear)3


beregond 25
warrior (shield) 6
ranger (spear) 3

also a question about the siege vets, is it 15 per unit or the whole crew that are manning the bolt thrower.

You should drop the Bolt Thrower. In those new scenarios, the enemy will clash upon your lines very early in the game and the Bolt Thrower won't be able to shot sooner than you expected.
You're extremely low on might, which means that your opponent will feel confortable to use heroic march to get close. At this low point cost, you should at least field as many troops as possibile or a powerful hero (something that, at low cost, can caught the opponent off guard) or, at the very least, Madril or Cirion.

If he can afford alot warbands he might mitigate the part of the enemy early clashes as he can avoid the hot zones after the enemy has deployed his entire force.

How he can afford that? best way I can think of its to add dunedain or rangers of the north as allies, get enough like 2 or maybe 3 so you can deploy them 1st and see where the enemy gonna deploy all his force, so you can safely deploy the bolt thrower to the points where it takes most advantage.

Keep in mind unlike mordor bolt throwers or the isengard ones, gondorian bolt throwers shoot is weaker as well as alot closer range.

The advantage of the Bolt thrower unlike those above is that a 24" S7 bow that shoots bettewn 1 to 6 shoots so all in matter its not that huge waste of points. But I highly recommend swift reload to get more shooting on the magazine mroe often.

The really bad side of it is that its unreliable even on a good day, unless you get good dice rolls, that thing aint that great as archers as for more bodies can fill up more or so the same roll but with better odds to land hits on something.