As the game is very story based almost all GMs will decide what is reasonable based upon the themes and backgrounds for the character. I know that might be frustrating when it comes to selecting an equipment list, but it is also a feature of the game style.
The starting Vis and Books for a Magus is controlled by the character's personal virtues and flaws, or they are designed as part of covenant creation. A magus typically start play after their apprenticeship with no significant personal magical resources.
A companion or grog would start with equipment approporiate to their profession, and little else - and like the magus character these equipment lists are discussed with the GM.
In more detail:
There are broad monetary wealth levels in Ars 5e which are reflected as virtues and flaws (Wealthy virtue ArM p.50, and other flaws like Branded Criminal or Outcast restrict the characer's wealth), and many are not available to Magi as they are typically wealthy by comparison in the setting.
The levels are Wealthy, Normal, and Poor, and there are also three costs for equipment for weapons and armour, which are expensive, standard, and inexpensive. Those levels should corrospond to each other. Note too that magical assets are treaded very differently from non-magical assets.
For non-magical gear I tend to allow:
A wealthy charater can take almost what they want within reason. At least 3-5 expensive personal items, and a large cache of standard or inexpensive gear if it makes sense (cloth and tradables for a merchant, lots of weaponry and arms for a mercenary captain, etc)
A normal character could take any personal Standard gear within reason, and a good sized cache of inexpensive gear for some story reason. They should also be allowed to have one or two Expensive items which relate to their background (eg. a good quality sword and armor for a soldier).
Poor characters should get almost nothing. One standard item, no expensive items, and nothing of real material wealth.
For magical equipment like Vis, books, spells, casting tablets:
They are all so "expensive" that they are basically restricted from being owned by non-mages during character creation, and starting Magi need virtues to have any of these things in abundance. Virtues and flaws like "Personal Vis Source" will dictate what vis and other assets a magus might have. With magic being the focus of the game, the magical assets tend to belog to the covenant, which is one of the meta-reasons why the Magi try to work together, as they need the resources of the covenant as much as the covenant needs them.
The undefined nature of gear in ArM isn't so much a matter of using house rules, it's what the designers wanted to happen. So it's actually rules as written and sometimes a little frustrating.