Safe pH range (acid vs base) for food

Ceviche is not exactly "cooked", but the acid causes the proteins to become denatured in a similar way. It may not kill all bacteria and parasites as effectively as cooking (with heat), so like sdg said, it's safest to use food that you would eat raw.

"Sushi grade" can refer to the fat content of the fish (like the USDA grades for beef marbling) rather than an assurance of its safety, so don't rely on that to pick out ingredients. Saltwater fish should be frozen at sea both for freshness and to kill parasites, and I prefer to get individually quick frozen (IQF) shrimp both for their freshness and convenience. Saltwater fish at your local supermarket has been previously frozen, so you're usually better off just buying frozen fish and thawing it yourself, unless you're really pressed for time.

Freshwater fish should not be used in a raw preparation, including ceviche.

I'm going to post a very general answer to this, since the question is rather vague about the application.

Since this was originally tagged molecular-gastronomy it seems reasonable to assume that this has to do with the use of some hydrocolloid for either a foam or gel. Unfortunately, it doesn't specify which one, and that's important information because they all have different pH ranges for hydration and gelling.

The following table of the most common pH ranges is taken from the Hydrocolloid Recipe Collection:

Substance              | pH Range
-----------------------+-----------
Agar                   | 2.5-10
Carrageenan            | 4-10
Gelatin                | 4-10
Gellan (Low Acyl)      | 4-10
Gellan (High Acyl)     | 3-10
Guar Gum               | 4-10
Methylcellulose        | 2-13
Pectin (Low Methoxyl)  | 2.5-5.5
Pectin (High Methoxyl) | 2.5-4
Sodium Alginate        | 2.8-10
Xanthan Gum            | 1-13

Looking up a bunch of different results for port wine, the pH seems to commonly be anywhere between 3.2 and 3.8, although every wine is different and the exact pH is going to depend on the exact wine (not just the source, but the individual bottle). The only way to know for sure is to use a pH Meter or some other type of pH tester.

If your liquid is outside the range for whichever agent you're using, then you need to use a buffer. If it's inside the range, you don't. Keep in mind that if you're at the extreme end of any of the ranges then you may need to use more colloid than usual. It's not necessary to be in the exact center but I believe it is best to be 1 pH unit or more above the minimum (or below the maximum in the case of an alkaline solution) for best results. Otherwise you might have to experiment with the exact quantities. (Note that it's possible that the HRC ranges above already include safety margins; I haven't done in-depth experiments so I can't be certain.)

Generally speaking, for many hydrocolloids the answer is yes, you will need a buffer for something as acidic as wine, assuming you're not diluting it with anything else. But don't estimate when you can measure; when making foams or fluid gels you need to be reasonably precise.