As with any airplane you can get dogs or good ones. Prebuy. Prebuy. Prebuy..... or you spend a multiple amount later to rectify the lack of one.
Pal of mine got himself a very used Seneca II in decent shapes, prebuy did not disclose any non starters. He's happy with it as can be, sais it is fast, reliable and refreshingly unsurprising so far. Value for money, I think the II is not the worst plane you can get. From the figures I've seen it is also pretty good in payload as compared to the later models. He's got the 123 USG tanks and the previous owner had new props fitted by STC with scimitar blades. Looks great and the performance is good too.
I've flown the I, II and III and would say that bang for buck I'd go for a good II anytime before shelling out considerably more for a III or IV without any substantial gain in performance or economy. IIRR the Lo Presti cowls are available as a mod to the II and those props are really lovely. The II has a range of around 850 to 900 NM, 180 kts @ 65% @ 23 GPH, payload with 123 USG of 370 kg if not downgraded to 1999 kg for Europe (which reduces the payload by 70 kgs). If you look at the book figures of the III and IV, there is maybe 5 kts speed at identical flow and 30-40 NM more range at lower payload, especcially at 1999 kg MTOW, because the Seneca III is about 100 kgs heavier empty. In Europe, a fully fuelled S III is legally a 2 seater.... Single engine ceilings are impressive, 16000 ft arounds.
The Seneca I has several negative points to it, which would keep me from buying one unless I have a very specific mission which fits it. My primary beef with it would be range and single engine performance. Its range is less than 500 NM with reserve and the payload with 93 USG on board (the S I does not have the LR tank option, pity in my opinion) of around 300 kgs. It will run at around 150 kts @ 22 GPH, so a whopping 30 knots less than the II at 1 USG less flow... Single engine ceiling is 4500 ft, which may be ok in the Netherlands but not very useful when flying in alpine regions..... Still, I remember from training that drift down is pretty slow. We usually managed to keep the I at around 7000 ft if we were coming down and around 5000 ft going up. Yet, if all you do is to fly the missus and kids from Shoreham to Jersey and like the safety of that 2nd prop, it may be the airplane which can do the job.
In today's market the SII might be a good entry level twin if you need the space and can afford the fuel. If not, Twin Commanche (Turbo if possible) for me any time if you don't mind
if you want a twin which runs at the cost of a high performance single.