The XTS line is no longer available from Motorola. You'd have to buy used.
It comes in VHF (136-174), UHF R1 (380-470), UHF R2 (450-520), and 700/800.
No free software can decrypt anything. Decrypting means taking an encrypted signal (one which is specifically coded to prevent outside listeners) and removing the encryption. A P25 digitally-modulated signal is not encrypted, it's just in a digital modulation. Scanners and SDR software like SDR#, HDSDR, and so on, can demodulate P25. There's no such thing as "decrypting P25".
And as for PRPD's P25 system being Phase II, from the screen shots in the early posts in the "Peel P25 Activity" thread, that's not the case. They have Phase II identifiers programmed, but so does nearly every other Motorola system. Identifier 00 shown in post #30 is for 851.00625, 6.25 spacing, and one slot (meaning Phase I). The frequency lists shown in posts #136 and #145 indicate iden 00 is the only one being used... meaning there's no Phase II currently on the system.
Anyway, to sum up: An XTS5000 will work "on a phase I system" whether it be a trunk system or a conventional frequency (which is, by design, always phase I). You will need to have specific flashcodes in it and they and the radio will cost lots of money. If you want encryption, you have to have the encryption flashcode, which is more money, and the keyloader hardware, which is even more money. And if you want to transmit on UHF you'll need a license (you can't just use FRS or GMRS frequencies).