you may ask this question to tomba or Mx-tifosi (direct via PM), but as far as I can see it, this place is o.k. within the F1technical.net BB.
Sure that in the endless www, there are other places/foren where people discuss such topics.
As for your question. The suspension will have a great influence on your overall design. As you will need to consider where,how and how much load you put/transmit into your chassis. So changing from one suspension typre to another is maybe not that straight forward, as you will need to keep the stiffness and overall integrety of your structure/chassis in mind.
As I said before, I´m not the kit car expert (far from), but as far as I know, or have seen in the past, most replicas/kit cars where based onto a VW bettle (fusca) floorpan (platefom). Just remember all the beach/dune biggy´s which where/are? still popular in Brazil.
The reason is simple, because the chassis/platfom is/was carrying all the load and holds the suspension, engine and gearbox in place.
There is not much load in the bodyshell itself, so you can go and put more o less any fibreglass body on top of it and don´t have to worry about, if it will be strong enough.
Some 365/spyder replicas are still based on this concept. O.K. I think, you need to shorten the floor a little bit, as the spyder has a shorter wheelbase then the Beetle.
We should first take a look at what you would like to do, I would say.
Do you want to modify an exsiting kit/replica o built a car from scratch?
What is your budget, at least the region 10 000$ 25 000$ or +100000$, as this will put some limits in place of what you can and can`t do.
Sure you can go out and built the best 550 Spyder copy in the world, it´s doable, but you may end up with a price for which you could go and buy a new Boxster, which still would be the better driving/handling car in all likelyhood.
A double wishbone suspension package is more then suficient for this type of car - IMHO. Keep in mind that most purpose built race cars are still double wishbone. I would not worry/spend time considering a multi link axle.
But if you go for double wishbone (which the original spyder does not have), you need to consider a full space frame chassis, as you will need the pickup points for your suspension (wishbones, dampers, springs etc.)