Having designed / installed etc around 1000+submersible sewage pumping stations in my 50 + years working live, few pointers.
1- Minimum solids handing size for your pump should be 80mm, unless you use a macerator pump, in which case the 50mm discharge would be fine.
2- Fitroy's cmments are correct "sewage" - means body fluids + wash basin + bath + dish washer + washing machine, NOT towels sanitary or childrens ducks otherwise etc, although dependant on how good your macerator pump is, it may handle them.
3- Don't fit a 2nd NSV but a gatevalve on the discharge pipe, out side of the unit.
4- Be prepaired for a smelly atmosphere should pump stop working and you need to open the unit.
5- Not sure if you tank has a high level alarm, if not have one fitted into say your kitchen - visual & audible ( with silence button)
6- Service & maintenace contract if your are not sure about working on tank & pump - IF YOU DO WORK ON IT SWITCH OFF ELECTRIC SUPPLY FIRST
7- Regarding lowering the sewer drain going over your neighbours land - If the drain has not been adopted by your local water company, then its between the two of you, But probable the whole length would need to be lowered, so from cost point a new sewer pipe might work out cheaper and easier.
8- Lastly make sure you buy a good make of pump. lots of what I call plasticy thinks around which dont last, speak in depth your supplier on what they offer.
A good electric submersible pump with maintenance should last 15 / 20 years.
Lastly if would like more info ask either here or via my email
Best of Luck : Robert