Is this an accounting system?
No, and it should not be treated as one. Xero remains the ledger, the thing your accountant reconciles and the record for filing. What this adds is the half a ledger structurally cannot hold: what the invoice was for, what the job cost, which site it happened on, and whether the contract is worth renewing. Do not run your books here.
Does it keep up with Xero automatically?
It can, on a schedule. Importing again updates the same table, matched on Xero's own IDs, and Keep in sync does that hourly or daily, which suits questions about months rather than minutes. A figure in a report here can still trail the ledger by up to a sync, and the import never writes to Xero, so anything going to an accountant should be taken from Xero, which stays the book of record.
What happens to invoices in a foreign currency?
They arrive with the amounts as Xero holds them. Where a business bills in more than one currency it is worth deciding early what a report should show, because totalling mixed currencies produces a number that looks authoritative and means nothing. Keeping a currency field and grouping by it is far safer than a single grand total.
Will my connection keep working?
Yes. Xero access tokens are short-lived by design, so the connection also keeps a refresh token and renews itself. That detail matters because an integration built without one appears to work perfectly and then dies about an hour later, which looks like an outage rather than a missing permission. Yours will not do that.
Can it create contacts as well as invoices?
The automation step raises invoices, and it bills a contact that already exists in Xero. In practice that is the right way round: a contact created automatically from partial information is the thing that produces duplicate customers in a ledger, which is far more annoying to clean up than adding a customer once by hand before their first invoice.