Judas said:
Is there a guideline to the CR's/ECL's of the party somewhere? And does that supposedly take into account possible magic items, etc. ?
The only help is the DMG chapter "Reward".
Don't try to be precise in evaluating your party, it makes little sense: you don't really need to think about their CR or EL, just get the average party level (6.5 I think) as a start, together with the number of PCs (6). This means that a monster with CR around 7 is probably the most appropriate.
A monster with CR 7 is supposed to be a "suitable" challenge for a party of 4 PCs at level 7 (or average level 7). Note anyway that "suitable" means EASY, at least easy enough for everyone to survive with a little effort and not more than 30% resources used, and obviously without bad luck. Increasing the number of monsters by 50% (or the HP of a single monster) raises the CR by one, so on the other hand being a group of 6 PCs they should easily afford a monster 1 CR higher. Probably most DMs would choose a CR 8 monster, or a group of monsters totalling CR 8 (e.g. 2 x CR 6). That would be a normal challenge for such a party even without the 10th level PC anyway, but I tend to be cautious: better an easy challenge too much than an unexpected loss of a PC.
A second thing to consider is the highest level of Arcane and Divine spells your PCs can cast (most importantly the latter): in your case, 3rd level arcane spells (Wiz6) and 2nd level divine spells (Cle3). Check that your chosen monsters don't go beyond your group spellcasting ability. For example, you can't restore drained levels with your daily spells, so be careful with undead, unless it's easy to buy restoration scrolls or potions.
That could mean also to pay attention to monsters with greater DR, if your party doesn't have good enough magic weapons, and none can cast GMW.
Finally, magic items can indeed raise someone's CR if they are better than the standard for the character level or for the monster entry in the book (otherwise they are already taken into account). But you'll have to figure out by yourself how much, if they are powerful enough to raise it. You can also ignore that and simply award more XPs if you gave the monster a special item, which of course raises the treasure by itself.
Edit: minor changes.