Since you want it to house goldies, go ahead and get the larger filter. I use an external pressure filter from Dr's Foster and Smith - the FishMate brand that is for a 2000 gallon pond even though my pond is less than 1000 gallons. Then you would need a decent pump, once again I got mine from Dr's Foster and Smith. A PondMaster Mag Drive pump. Other than that, not much. You could even go with one of the waterfall type filters - that way you would have a perfect way for the water to get back into the pond and it would add a bit of extra aeration as well as a place for some floating plants like water hyacinths and water lettuce. These plants will help with the biological filtration also.
I like the shell since it is already rigid and holds it's shape. The liner gives you a bit more flexibility as to where you have your pond. The shells can spring a leak as easily as the liners. My first pond (shell) has two holes in it. One on the shelf where a rock fell in from a cat knocking it off the waterfall (left a hole big enough to stick a pencil through), and one in the bottom just from wear and tear - the fiberglass just wore through. My current pond is a different material. The first pond was fiberglass, this one is a sturdy plastic or vinyl. There are really only three things you need. The liner, a pump, and a filter. Anything else is just window dressing. You want to go as big as you can with a pond - especially since you want goldfish in it. A larger filter and pump means less wear and tear since they don't have to work quite as hard and less maintenance for you. The larger pump will also give you the flexibility of how you want the water returned to the pond - whether it be a waterfall, two waterfalls, a multi-level waterfall, or a fountain of some type. Or any combination of them. Look at what is in the kit, if it tells the brand/type of filter and pump and such, google them and see what they would cost on their own compared to other filters and pumps and such. Make yourself a chart that includes the brand name, the price (if purchased separately), the pond size they are able to handle, the flow rate, and any other relevant data. Then sort them by size, by price, etc. and look at the list very carefully. Ask yourself why does filter A that is rated for a pond up to 4000 gallons cost $425 when filter B that is also rated for a pond up to 4000 gallons only cost $270?