Looking to do a coolant flush/thermostat few general questions

[NYJets]

Yest driving under normal conditions, my water temp gauge went berserk and I am now scared.
2005 WRX - minor mods
Car has 88k and i dont know the service history.

I am going to do a full coolant flush and thermostat change.

Ive been searching and see to 'burp' the system - i understand this and will do...obviously.

My questions-

What temperature thermostat is stock, and is there an advantage to a cooler one/disadvantage?
How many gallons approx does the car take(no service book)?
Is it necessary to use Subaru coolant, or can I just use a high quality basic coolant?
Any other tips/tricks/etc I should know before I undertake this?

[NYJets]

I am getting a very similar problem as this other member from this thread,
Fluctuating Coolant Temp

A few posts below where is says Fluctuating Coolant Temp...im thinking thermostat....

[SD_GR]

The stock thermostat should be open by 80C according to the manual. By 90C the fan will be on low speed with the A/C off and by 95C, high speed. Both fans will kick in with the A/C on, and the fans are also speed-controlled depending on whether the car is in stop/go traffic or at high speed cruise.

Capacity for the 2002 model turbo car is roughly 8L. Subaru now sell green coolant, that will require dilution (use distilled water) or blue, that is pre-diluted (so buy two large containers of blue if you decide to use that).

I see no reason to use unauthorized parts; the Subaru stuff is readily available and not all that more expensive (a few dollars I suppose). There have been a couple third-party thermostats sold but I've not used them and they were several times more expensive than the correct part anyway.

Use a stock gasket for the thermostat housing. Do not overtorque the bolts. I think the spec is less than 5 ft*lbs but check to make sure. The thermostat must be oriented correctly in the housing; look at the old one and replicate that exactly.

Subaru also sell a small bottle of coolant conditioner and I have heard they require its use, so ask about that too.