A 5 string makes a lot of sense if you need to go down to 'b' or 'c' vs a 4 string 'c extension'.
Modern orchestral music requires those lows notes.
Watched the Vancouver Symphony bass section up close the other night. Out of 7 beautiful basses 6 had the 'c extension' and one was a 4 string without it. Makes me sad to see these fine scrolls 'modernized'.
North Americans tend to have the extensions while Europeans choose 5 strings.
A lot of basses about a hundred plus years ago were strung with 3 gut strings. There are old 3 string bass method books out there. I recall my teacher (about 40 years ago) lamenting how many big sounding smaller basses were strangled when converted to 4 steel strings. Some folks play 3 string bass today.
The old wood tuning keys can split. Some fantastic all metal key sets are available today, quite expensive of course. Should be good for 200 years of daily use

Graeme, nice looking 5 string! With those sloping shoulders you should zip around the thumb positions.
My Busan copy 7/8ths bass with big round shoulders requires special tactics to get up high. I have another 5/8ths bass that has sloping shoulders plus a big sound through the thumb positions. Curious that my large bass sounds big in the low registers and light in the high registers and my small bass is just the opposite!
Cheers