John:
I've no experience with the N4 but the N3 is not so difficult to rig but it is very time consuming. The 10 l'hoteliere fittings (6 of which are completely "blind") take time and patience to connect and safety becuase the fuselage ones must be done by "feel" . Not a difficult glider to fly compared to other open class gliders. No issue with winch launching assuming that your winch and driver have experience with heavy gliders. Gravel & grass runways are fine assuming you have a tow plane that can accelerate smartly. Be careful in tall grass - especially if you are using a CG hook for aerotow.

All N3 & N4s with motors are extraordinarily tail heavy when ground handling. Some lifting device is helpful. Wheel braking on the Nimbus is poor. Try to get one with a trailer that allows you to keep the tip panels on in the trailer (less assembly and taping).

Many N3 owners (including me) changed 2 things: 1) Disconnected the bungees that run to the outer panel spoilerons (which were useless anyway) as this reduced the aileron control force without adverse impact on control, and 2) ignored the directive to completely fill the outer panels with water before filling/using the inner panels. Having about 40% water in the outer panels and 60% in the inner panels helped the roll rate significantly.

I agree with the others that the ASH-25 is a nice glider also. I think it handles a bit better than the Nimbus and has the flap handle up on the gunnel where it belongs. I did not like having to stare at the floor to find what flap position I was in (which you must do with the Nimbus). Both are well supported by their factories.

Good luck.

ROY