Here's my take: I put nitrous on my car with 143k on it. I started on the 75 shot and removed 10% timing (which MAY have been a degree, or 1.5 degrees) and also added fuel with my Diablosport.
The car has yet to miss a beat with it. I put in 1 step colder COPPER Autolite plugs. I STRONGLY recommend only using copper plugs. I can't remember if it's iridium or platinum, but one has a negative chemical reaction with the nitrous and it can get ugly, fast.
The biggest indicator you can have as to whether your engine is going to take it or not, is to do a compression test and to make sure your fuel pressure is where it should be. Also, read your plugs how they are now. If they are good, that's a sign your engine is running properly. If you pull bad plugs out (indicating a problem), you are asking for failure if you don't fix whatever is wrong first.
If you have good compression and your getting fuel to the engine, a 75 shot is pretty damn safe.
Aside from that, a wide open throttle switch is the very least you can do in terms of safety. You should also invest in a window switch (or you can flip the arming switch on at WOT when you are ready to spray, and risk missing a shift and hitting the rev limiter).
A purge kit is nice, but I have not used one so far and it's not my top priority to get one.
I can't spray in first on street tires. It will just spin. On a set of BFG drag radials, it still spins a bit but it will hook enough that I can still spray in 1st. This is partly because of my suspension and gears, though. I can spray in second on street tires, but depending on the road surface and temperature it will sometimes spin as well.
Like Tim said, don't risk it below 3k. The lowest I have sprayed was at 2800RPM and it was fine, but that's not far below 3k.
As far as a bottle heater, if it gets hot where you live the bottle will heat itself on a warm day by sitting in the car. Mine so far hits between 900-950psi on a mid-upper 70's through low 80* day, and in the summer I'll be worrying about it over-pressurizing.
In the fall/winter, a heater is an absolute necessity to achieve proper bottle pressure.
I mounted my bottle in the trunk, at an angle right behind the drivers side rear seat. I can reach it going down the road and turn around and see the PSI gauge. Keeping it at a rearward angle allows gravity to feed the nitrous into the pickup tube in the bottle as well, although mounting the bottle straight facing the front of the car will give the "best" results.
I ran the feed line through the interior, under the carpet, out the kick-panel area on the drivers side, through the fender, and into the engine bay. The bottle is ALWAYS off when I am not using it, so if there is ever a leak in the feed line, it won't be there when the bottle is off. And the line is under so much pressure when the nitrous is on, there's a decent chance you'll hear a leak if one develops. If you want to be the safest, run it under the car. I didn't do that because I'm worried about snagging it on something going down the road.
You'll have fun with it, definitely.