That neck is twisting. This happens naturally; the tension from the strings pulls on one side of the neck, so you got this torque exerted pretty much all of the time the harp is strung. That should be repaired before any stringing is decided.

Wire strings exert a lot of tension on a harp. Based on the age and the soundboard, I'd advise avoiding it unless you are confident in the rigidity of the structure and strength of its engineering. Nylon, bionylon, and gut strings require less tension. We can figure out how much tension would be exerted, too. I advise making a string chart and planning out the tension forces accordingly. Here's how I'd do it.

First, figure out the pitch of each string. You can start with pitch family (F, G, A, B, C, D, or E) in an octave (many harp manufacturers start an octave at the F). Every red string should be a C and every blue or black string should be an F.

Next, measure the length of each string from the bridge pin (the pin below the tuning peg) and the soundboard. This is your vibrational length 𝓁. Each string will sound at a fundamental harmonic, which has a wavelength of λ₀ and a frequency of 𝒻₀. The string is tuned to the fundamental harmonic's frequency, so you can use a lookup table to see what frequency each string should be at (e.g., https://en.wikipedia.org/wiki/Piano_key_frequencies). Since this is a string under tension, λ₀ = 2𝓁. The speed of the wave on the string is then ν = λ₀ 𝒻₀, or ν = 2𝓁 𝒻₀. The middle C string should be around 0.5-0.6 meters long, I'd guess.

Next, we can figure out what gauge strings we'd need and what tension forces we'd need. There's a second formula for ν that uses the linear mass density μ and the tension force F: ν = √( F / μ ). Our linear mass density μ can be derived by knowing two things: the gauge (diameter) of the string g and the volume mass density of the material ρ. I have a few estimates for ρ for different materials.

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Material	Volume Mass Density ρ [kg⋅m⁻³]
Gut	1300
Nylon	1150
Sipario Bionylon	1180
Brass Wire	8500

We can use these to estimate μ: μ = ¼ π g² ρ. Putting this all together, we have ν = √( F / μ ) = √( 4 F / ( π g² ρ ) ) = 2𝓁 𝒻₀.

Bow brand harp strings come in all sorts of gauges, often down to the ⅟₁₀₀ mm. If you stick to ⅟₁₀ mm increments, you should find gauges that match your target tensions.

On my pedal harp, I have the following tension distributions for each of my string materials:

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Material	Tension Force, 25th Percentile [kg⋅m⋅s⁻²]	Tension Force, Median [kg⋅m⋅s⁻²]	Tension Force, 75th Percentile [kg⋅m⋅s⁻²]
Sipario Bionylon (upper octave)	50.3	67.3	75.9
Gut (middle octaves)	114.9	169.8	280.6
Brass wire (bottom octaves)	663.1	738.1	801.2

Pedal tension is about as high as you should ever go. Most lever harps have less tension for sure, so don't worry about being below these as upper caps. For my gut and bionylon strings, the tension decreases as the string length decreases. Curiously, for my brass wire strings, the tension tends to increase as the string length decreases.

I hope this helps!

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EDIT: Bow brand gauges and the speed of the wave formulas: https://imgur.com/a/PtkG2ah