Wait. I can do 3 first level spells a day. How many do I know?
Any that are in your book- standard for starting at first level is all cantrips and 3 first level.
Sensei earlier said wizards start with 3 - none extra for INT?
Going by the Player Handbook, Wizards get their Int Bonus in additional starting spells, so 3 flat might have been either an oversight or an intentional DM alteration.
Also I realised I didn't need to buy a spellbook. Any idea what I could retrospectively spend 15gp on?
Rope? Waterskin(s)? Food? Sacks? Chalk? Non-spellbook paper? A cover for the spellbook? More ammunition? A quarterstaff?
Also, you may want to grab a sling or two. They're free, and can be very handy; anything from letting off a few rocks in a pinch, makeshift bandages, muffling metal (I like using them as bell clappers, myself) or more.
Wizards can scribe scrolls. How does this work? Do I need ink other than basic ink to scribe magic onto scroll or book?
"You can create a scroll of any spell that you know. Scribing a scroll takes one day for each 1,000 gp in its base price. The base price of a scroll is its spell level × its caster level × 25 gp. To scribe a scroll, you must spend 1/25 of this base price in XP and use up raw materials costing one-half of this base price. Any scroll that stores a spell with a costly material component or an XP cost also carries a commensurate cost. In addition to the costs derived from the base price, you must expend the material component or pay the XP when scribing the scroll."
Ah. I thought that was for spells cast per day.
It is.