I ran a older shamrock 20 for several years. Good boat. The old ones (this was a 1976 hull) were single skin. Later versions (starting in the 80's) had a molded inner hull liner that really increased the weight.
Ours had a ford 302 v8 with two-barrel carb that was rated at 175hp, 1:1 borg gear and 13x11 wheel. It ran about 20kts at 3000 rpm, topped out (if I remember right) at about 28 kts at maybe 4200.
With a bunch of load in the boat, it felt like the engine had plenty of power, but not too much left in reserve.
I think a tdi at 120 or so hp will plane it out ok with a light boat, but with any load it will be underpowered and engine will be running at a real high load percentage, something not seen on cars.
The 17 should be ok. It came with a mitsubishi 2.6 that was probably around 120-150hp.
Been there, done that. 18' Privateer with 45hp 1.6 vw idi. 20kts @ 3400, 23kts @ 4200