for Biohacker or Mystic/Biohacker

First off, the biggest hurdle to multiclassing, IMO, is weapon specialization. Unless you take three levels of the same class to begin with, you're gonna be behind the damage curve at third level by a significant margin unless you're sticking with small arms and operative melee weapons. I'm not taking about a 20% bump, either. If you're dealing 1d8 damage with an azimuth laser rifle (popular choice), weapon specialization takes that up to 1d8+3, which is a nearly 70% increase in average damage.

With any multiclassing you are pretty much required to give up your 3rd level feat to grab specialization in at least 1 weapon type. You fix this later at level 5 with the mnemonic editor once you can qualify for versatile specialization. At that time you take versatile specialization as your 5th level feat and edit your third level feat to something useful. It's not ideal, but it is workable.

starting gear suggestions

everyday items

New SFS guide came out today. You can officially buy single UPBs now, THANK GOD, so my list is legal.

all Armor automatically has a Personal Comm Unit

new player guides, and playing online

to attack Incorporeal creatures and bypass DR/magic

advice for new players

"Further, no spell of any kind is required to make magic items in Starfinder. If you have ranks in Mysticism equal to the item level of a magic item, and the GM considers it to count as an item that can be created, you can create it. Your race, class, spells known, and even spell casting ability do not matter."
But crafting is useless unless you have no access to stores.

yes, it's worth it and expected that you'll get some

Spells from Connections are in addition to Spells per Mystic level.

Starfinder assumes that you will be using augmentations; if you're not using a portion of your wealth on cybertech and the like, you aren't playing up to your potential.

You really don't need to upgrade your weapon very often; I was fine with a level 2 pike until I could buy a level 7 yellow star nova lance, for example. Armor does require upgrade every 2 levels or so if you are going to be taking any heat at all, but that still leaves plenty for augments.

Completely aside from the Personal Upgrades (which are practically mandatory), you get augments to shore up weaknesses, free up gear slots, and play to your strengths. It also makes for great roleplay.

The trick is you don't want to get the +10 foot speed to legs, and then upgrade to the 20, and then upgrade to the 30, because then you're just burning cash. Pick one, wait for it, and get it.

(No, Second Skil is not enough for combat after level 2, but keep it for social situations where visible armor is inappropriate.)

You can upgrade Personal Upgrades by just paying the difference in cost, but you can't upgrade any other augmentation without paying full price. You can's even sell back your old one, because it is keyed to your genetics.

You can replace an old upgrade by just buying and installing a new one.

Biotech is overall worth buying for unique biotech items.

Adaptive Biochains can be worth buying for flavor.

Biotechnicians get discounts on biotech.

The benefit of an environmental outfit can be added to another type of clothing (including a different environmental outfit) by doubling and adding together the price of both types of clothing.