Knight. Period.
P/K is the most versatile of the priest combos. It's a pure healer/buffer, and the combo of p/k/m will let you quest without too much trouble.
Acquiring gear and stats for a healer is easier than falling off a log. I've geared a healer and a half to endgame, and it was stupidly easy and cheap. Because you don't rely on stats for healing (Healing Power Increase Points are what increase your healing and they are built into your gear and on your weapon), you can feel free to stat your choice of wis, stam, def, hp and even mdef (for a more pvp build).
Invest in a good weapon. P/K should go wand/shield. Depending on which server you are on, ToSH and KBN healer wands should be pretty freely available, assuming you're leveling quickly. A lot of healers use combinations of shell, proof, mem and instance gear- choose what you can get cheaply and overdura.
I personally used X stats from Chrysalia on my druid. I actually buy stats for my p/k. Both are currently healing KBN hard and Bedim. I probably spent less than 2k dias and less than 100 mil for the two of them, total. That's pretty cheap for an endgame viable toon. Neither are going to win any epeen contests but they get the job done.
You can farm mems, Energy of Justice, Phirius shells and gold for your gear and stats. Farm gold for the item shop plussers, rune drillers and runes. It's very very doable. Healer is easily the most cost effective role in RoM.
Oh, and people may try and talk you into some of the more esoteric priest combos like p/wl, p/r or p/s. All are great- and you could argue that those combos have better party buffs- they do. The problem is they don't have the built in survivability that the p/k has- and as a free to play player with low resources, that will allow you to heal more in less gear. Those combos are perfect for people with money and time- we have a p/r in guild who's beastly beyond belief and he's single-handedly restored my belief in the combo- and he's also spent a few thousand bucks on his gear.
And he still doesn't have the survivability of a p/k who spent $20. And his third class is knight.
Keep it simple. P/K is simple as it gets. If you do decide that p/wl or p/r or p/s is for you, don't go mage. Mage does help a priest somewhat with leveling, but you can get by with the class-specific skills if you don't gear like a priest. Gear like a mage. Get mage gear and slap some dirty int/ma or ma/hp stats on it, and go on that way til you're actually ready to gear for instances.