Firstly your friend needs to work out which market they are targeting as that will influence what is important. A property that size is ideal for large/ extended family breaks or hen/ stag parties.
I've stayed as a hen at a "rural luxury" large property. There were 10 of us in all. The hot tub was very popular with us, also plastic glasses for outside/ hot tub use (especially for the hen/stag market). Plenty of parking was available and was needed as the guests were all geographically spread out so only the bride/ sister of the bride and mother of the bride could share a lift. If the property is rural clear instructions/ what 3 words/ accurate location information is really helpful. Generally for hen/ stags they are likely to want to be near pubs/ activity centres. Have ample parking or be close to or have good links to public transport. Local taxi services are also really helpful and if the property relys on very small taxi firms that are likely to stop running at 8.30pm/ only run Friday and Saturday nights and have to be pre-booked weeks in advance then alternative taxi firms or notice that this is the case would be great. Beds and bathrooms are another consideration, twin beds are great in places that cater to hen and stag parties (Not all guests at hen/ stag parties know each other really well and be comfortable bed sharing). Plenty of bathrooms - 10 women getting ready is a long process.
If your property has neighbours, its probably not a good idea to market to the hen/ stag market as they are generally loud/ early hours affairs (we were still drinking in the hot tub at 3am) and neighbours will complain.
I've also gone on extended family breaks (Me, DH and DD, DM and DDad, DB, SIL and DNs). Parking once again is important to us, we generally bring 3 cars. Extended family breaks are generally going to have more than 1 set of adults staying so more than 1 "double" room is required. There is also likely to be children so twin rooms/ rooms that can be double or twin. More than 1 bathroom and 1 downstairs toilet.
In terms of what attracts me and my priorities, I only book places with at least king size beds. Off street parking and if staying with extended family in a large place i expect there to be ample parking - it just isnt fair to neighbours to bring 3 cars and take up all the limited on street parking, A dishwasher and ample kitchen to cook meals for the amount of people staying. A large place should have more than 1 tray, 1 frying pan, 1 saucepan etc for cooking. Plenty of glasses/ cutlery. On that note I don't want a kitchen of cast offs and what was obviously great granny's old oven dishes. If it is marketed as luxury then I want to see matching crockery that is well looked after (not full of chips/ a combination of 2 not quite matching but close enough sets). I personally like either a double size shower or an over bath shower - I don't want to be cramped in a tiny box shower. Heating that works/ heats the place to a decent standard. I have a toddler so if catering for children then high chairs, stair gates and travel cots are much appreciated. A decent information book that is up to date with local recommendations is a firm favourite.
A massive deal breaker for me and will lead to me leaving a negative review for: I DO NOT WANT a long list of jobs I have to do before check out that really should be done by the cleaner (and I have paid a cleaning fee for them) at the end of the stay. I am on holiday, I can strip beds and take duvet covers off quilts, hoover behind and under furniture, mop floors, defrost and clean out the fridge and dust at home for FREE.