Hey Tommy, welcome aboard.
For insurance, basic third-party liability insurance is what's required.
You can find a decent pre-built car in the $5,000 to $10,000 range. Check out
www.specialstage.com or our own classifieds. Cars pop up from time to time.
Building your own car is almost certainly going to cost more money. The bits and pieces add up quickly - things like skid plates, odometer, intercom, etc. etc. all cost money. Buying a second-hand car is definitely more cost-effective when you are starting out. And try a 2wd or naturally aspirated 4wd car for your first ride, they're much cheaper to run. The Volkswagen Golf and Subaru Impreza 2.5RS are great starter platforms.
Suits can be found for around $350 new or less for a second-hand suit. Helmets start around the same price.
Event costs vary. For a single-day event like Galway, you should budget for a set of tires, towing costs, fuel and entry fees. Probably around $1000 to $1500 if you're buying brand new tires. An event like Tall Pines will require a couple of nights in a hotel, food, and more gas and tires.
You'll have to also factor in a parts and wear-and-tear budget.
Hope this helps.