FWIW.
New kit 'plane "the Sling" made in South Africa has a 4 seat version using 100 Rotax horses they also do a two seater. One has gone round the globe. In the U.K. both (I think) are approved by our LAA as Permit operated a/c.
Likely with 100 h.p. you will get the rough shut down, but I understand the "soft start" ignition module retards the firing till the engine is running properly so should be smooth. Even so 100 divided by four occupants has to mean two adults, two kids only and watch the fuel load then.
I run behind the much gentler 80 hp Rotax, smooth start & stop, enough power to propel a self maintained LAA supervised Rans S6-116. Cruise might be too slow for you at a modest 98 mph at sensible rpm. Consumption 14 l/hour. Range if you have the bladder is 4 hours plus an hour reserve.
Flying BTW, in the UK even in the 'crowded' south is pretty relaxed & can be mostly non radio and simply go overhead or route round most controlled air-space. No flight plans necessary, no permissions & one can still take off from one's private strip and e.g. go direct to France where they want you to clear at one of their 'Customs' aerodromes. Then coming back is the same, no need to land till overhead one's base strip.
Insurance (third party) is compulsory, costs about half of that for my motorbike. LAA and inspection some £300 per annum, all other work is dead easy to DIY. MoGas from a supermarket is today £1.2 per litre X14 = <£17 per hour.
All depends if your proposed flights are too long for that type of plane but for fun, security and hours in the air it's hard to beat.
mike hallam