I used to work in Travel and some of the cheapest times to go were begining of May, most of June. Beging of October and December, most of November and January.
If you go long haul most of your charters will charge you quite a bit to upgrade which will eat away at your budget. If you go schedual then you`ll more than likely get a large enough seat pitch that you won`t need to upgrade.
Have a look at aircraft seat pitches as some vary alot, our friends have just been to Sharma el Sheik and they went on a Jumbo jet which was chartered to Mytravel (Air Atlanta) they only went hb but you might be able to get All inc in your budget.
If you look on Eclipse, Direct holidays and Portland they have some good offers, they should also list who they are flying with so you can check their seat pitch.
Which airport do you want to go from.I`ll have a look and I`m sure others will for you.
Christine W