Jumping right into AG?

There may be some truth that stouts are more forgiving than a lighter beer, but they are not bulletproof. I think you should brew what you would like to drink, but to make it easy on yourself, avoid something high gravity, unless you just don't value drinking beers that are low or moderate alcohol. High gravity fermentations can be challenging.

 

The term Spring Water doesn't tell you anything about the mineral profile of any random Spring Water you might buy. It's as much a crap shoot as using unknown tap water.

It's possible to get lucky, if the unknown water is a reasonable match for the style being brewed.

 

My point about avoiding ruining your beer through water modifiers is not that spring water would be ideal. Simply that you can avoid making critical mistakes when on your first brew day.

Mastery of being process and dialing in your setup should always be forefront for a new brewer. Definitely NOT brewing water. Besides, fermentation process has by far the greatest impact on the quality of your beer.

Back to my original point, if the water tastes fine, most likely your beer will be fine. Filter tap water is likely fine to start. Most bottles US spring water is filtered/reverse osmosis with additions for taste.

 

I'd suggest finding out what's in your water and then modifying it for both mash pH and to fit the style. This might include treating with Campden to remove chlorine and/or chloramines. Some filters remove these and some don't. You can modify your water by adding salts/acids and/or by blending with distilled water.

Some people, including me, have tap water that's not really good for many styles. I start with distilled water and build the profile I want by adding salts/acids. Some people use Reverse Osmosis filters to make their tap water close to mineral free, and build from there.

This is all pretty challenging for a first time brewer, which is one reason it's probably better to get a few extract batches (using distilled/RO water) under your belt first. This works well because the extracts already have the minerals from the manufacturers' mashes. You won't be controlling the (mineral driven) flavor profile this way, and it's one of the drawbacks to extract brewing, but it's better than doing an all grain batch without paying any attention to mash pH and possibly ending up with a beer that's astringent or too dark, for example.