my own answers:
1- you can always hire someone to install softwares on your site... and Photopost always offer installation for usual fees.
2- how many pictures can your host let you put in ?!... it always depend on size of the files, compared to the webspace you pay for (usually, a 50Megs webspace can accept between 100 to 500 photos)
3- one of the goals of PP is to let each user have his own album, like you see here: http://www.photopost.com/photopost/s...rs.php?cat=500
... and you as the admin is the one to set privileges regarding space and number of files used by each group of users...
also, your forum can be directly connected to PP because of the forum integration part... sharing users table from your forum.
4- for minimal knowledge of HTML, you can modify the "templates/look" of the tool... or you can hire someone to do the job... on http://photopostdev.com
, you will find some good resources for this.
5- better to integrate PP to your vBulletin, for one reason: users will not have to register twice, and you will have the same usergroup privileges on both.
6- hotlinking as you suggest, is not good.. but photopost have some external feed scripts to let you integrate these resources inside vBulletin, look at http://photopostdev.com
for example (the entire list of PP softwares is integrated with vBulletin there!)