What questions should I add into this form?
Chances are your gut already answered that for you. Trust your instincts. You should ask everything you need to know about the specific client or project in order for you to appropriately assess whether or not your company is interested. Try not to make the template excessively long, keep it to the point. Both parts will appreciate that. Also, don't ask weird sh*t and be nice!
Is the form customisable?
Hell yeah! That's the whole point. The form wouldn't be half as effective as it is if you couldn't tweak the questions or add some new ones to result in a way more proper fit for your interests. Moreover, almost everything is modifiable: the fonts, the colors, parts of the design... what else do you want from us? This would be an adequate time to quote the farmer guy meme. The thing is, this is honest work, but... it ain't much? Man, it's more than enough!
On a side note, you'd likely find it interesting to know that we there is the possibility to not only bring the creative part of your brain to the table, but also to synchronize the form responses with your CRM and to include CC payment options in the form itself. Thank us later.