I have used some software organs, but it's been a while and I'm not sure I remember details.
Hauptwerk of course is the top, and it isn't as demanding of computer resources as you'd think. It has free demo versions, so try them out. I got it to run on a relatively old laptop. I used one keyboard, one midi-usb interface which included a sound card, and I ran RCA cables to a home stereo with decent but not audiophile speakers. It worked pretty well. If you're doing multiple manuals and a pedalboard, and running lots of stops, you might need more computing power. I made one bad mistake, I let our regular organist play it outdoors where we needed some volume. I programmed the top few keys on the keyboard as registration keys and told him not to touch them. Of course he did and messed everything up.
There are two other software organs I've used, and both used less computer memory and were more intuitive to learn. Hauptwerk is so feature full it's a little intimidating.
J-organ worked well, I think that might be grand orgue now. And Miditizer was the easiest to get going, and was okay on the old laptop. But it's a theater organ rather than a church organ. Still, I'd give it a try, if for no other reason than to be able to experiment with MIDI and sound cards.
Getting the software to work isn't the hardest part. Any cheap keyboard has MIDI out, but you have to get MIDI into your laptop. I use a Fastrack Pro, which is an inexpensive USB-Audio interface that takes microphone input, MIDI input, and has soundcard output. Then you need an amplifier and speakers. I use a regular thrift shop home stereo. That won't fill a huge church but it's fine for most smaller areas.
Oh. Be SURE to disable any kind of power management. You don't want your laptop going into sleep mode during the sermon, then discover you have to restart all your programs in the proper order right after. I prefer myself to be in sleep mode at that time, not the computer.