Cool concept! There are a lot of colonial houses around New Zealand that might suit, just a matter of finding something available to rent in your area that suits your budget. Getting people to dress up 20s or Victorian and to stay at a historical venue will probably help get them in the mood.
As folks have said, a full weekend may be a big ask. It takes a huge amount of activity to fill a weekend, much more than you’d think if you haven’t run a larp before, especially if it’s a player-vs-environment type game where the players are basically responding to stuff that is thrown at them. You don’t want people sitting around twiddling their thumbs, and if your players aren’t larpers or actors then they probably won’t fill blank hours by roleplaying with each other in character and creating conflict with each other (even tabletop roleplayers aren’t in the habit of doing this).
One of the mainstays of Cthulhu stories is that there is a principle investigator (the narrator), and over the course of the story his companions die around him, showing just how dangerous the situation he’s in is. Something in that direction might work quite well for your event, as you’ve got the groom as a central person. If you wanted your event to be more scripted (and therefore easier to run) than the average larp, then you could plot out a series of nasty events that will happen to the groom’s companions over the course of the event, building up the tension. The people who are dying would be in on it, so that it could happen dramatically and you could use special effects (for a simple example, they could chomp down on a blood capsule and have foaming blood come out of their mouth). Those who die could then help you portray whatever the monstrous thing the groom is up against is. Probably best to leave the groom with one or two companions right to the end if you go this way though, being left with no friends on your side at all wouldn’t be fun for most people.
I’ve run a horror game in this style once. The pre-briefed players who died off ended up as zombies, and as the game went on the surviving players were more and more surrounded by zombies who they had to lock out of the house - it became quite survivalist. One nice feature of that game was a seance early on. One of the planted players forced the ouija to spell out hints about the mystery, and then he got “possessed” by the spirit and did the blood foaming from the mouth thing. The players had to figure out the mystery of the house that was causing people to become zombies and fix it, all the while fighting them off. The key to zombies is having someone do zombie makeup for them when they turn (take them away somewhere to do it), and training the players to walk and groan in an appropriate manner.