I don't mean to sound ignorant but you've obviously not looked into standers. Your gonna spend $5000-$6000 on a 36" stander, you might be able to get a used stander for $3000 in good condition. Stay away from Toro Grandstand's that are 2009 models, it was there first year and had MANY issues.
Yes you should buy a brand new 36" walk behind. Belt drive is in your price range. I can mow 12 1/4 acre lawns a day with my 36" Toro walk behind which is belt driven. I do not use a sulky.
I bought out another company this season which has large 1/2-3/4 acre homes and several large 5-6 acre areas. I will be upgrading to a used JD 797 with a 72" deck as well as a 52" stander, I haven't chosen my stander that I want yet. I really like Scag but I would like to match my equipment so I would prefer a Toro or JD. It will come down to me re-demoing equipment this March.
There is an Old Blue Chair for each and every one of us, we just have to find it.
2006 GMC Sierra 2500hd crew cab
BOSS 8'2" Power V
Hustler X-one 60"
Toro Proline 36" wb w/t-bar controls
Shindaiwa T254, Echo trimmers, chainsaws, pole hedge trimmer, pole saw, hedge trimmer
Shindaiwa 633 and Echo 760LN