To begin with, only the DAKAR rally (at least at present) has imposed the 450cc limit... and that is ONLY for the ASO list of "Elite" riders (a list of the 20 - 30 top rally pro's and expert level riders as determined from the ASO/FIM rankings over the last few seasons). All other riders are still permitted to use open capacity machines up to 700cc (super prod) or 900cc for marathon (standard) machines.
As to the question of a 450/530 EXC or the 690 E for an amateur rally like the TUAREG.
: a lighter more nimble machine, considerably lighter to ride in heavy dunes... or on the more technical enduro like terrain; like exist on the first and last days of Tuareg rallye or
the more enduro/terrain like navigation enduro events like the Albania rally and Italian moto rally championship events.
There is a number of rally kits available for the 450/530 KTM machines... and for an event like Tuareg (160 km max refuel distance), a suitable bike can be easily built with a 14 liter accessory tank, a simple roadbook holder/tripmeter with standard enduro light mask, a sturdy sump guard/water tank and perhaps an auxiliary 5 liter fuel reserve tank on the rear (for safety fuel margin).
: The comparatively short service interval typical of the 450/530cc enduro based machines (smaller engine oil capacity and higher state of performance/tolerances; valves shims, piston rings etc.) means that in standard trim, the EXC will be a more maintenance intensive option for a privateer both timewise and economics. (Assume that if you plan on doing own
mechanics = less time in bivouac for eat/rest/sleep OR
you have the extra cost of taking a service personal). Of course it is possible to fit extra oil cooler/capacity to the EXC engine... but this involves more complexity and cost... maybe not ideal for amatuer levet participant?
There is plenty of rally kit accessories for this model. Similar to the EXC; by using an accessory front tank (in addition to the stadard rear subframe tank) the 690 can easily be made to comply with minimum fuel capacity for amatuer rally event. Same simple solution is possible for the roadbook/tripmeter mounted to the handlebar with original light mask, and a rally sump guard/watertank. Simple, cost effective.
Another big advantage of the 690 E is the longer service intervall of the new generation LC4 engine. Lower state of tune (compared to the EXC machines), larger oil capacity, means that (realistically) you could do an event like Tuareg with minimal engine maintenace (daily airfilter changes and maybe one oil/filter change mid rally) which could equate to between 10 to 15 hours spared in service time during the course of an event for a privateeer rider doing own service = valuable rest/recovery time perhaps, if is your first rally.
The biggest drawbacks for the 690 E is (as you say) the extra weight (compared to the EXC) AND the limitations of the 690 E suspension components... in which case the 690 R would be a better choice
, as it has more competition inspired suspension AND all rally kit components (tanks, navigation equipment etc.) is same as on 690 E.
Summary: If you have access to a mechanic; go with 450/530 EXC solution... If you plan to do your own service and looking for a bike with longer autonomy/less obligatory service/maintenence, perhaps the 690 E/R solution is better.
I have a Swedish friend who is building just this type of bike for his first rally - the 2011 TUAREG Rallye... A 2010 KTM 690 R with Rally Raid UK front accessory tanks (2x 5 liter capacity twin front tanks on either side radiator shroud) and KTM hardparts roadbook bracket, MD roadbook roller and ICO tripmeter. Simple, practical and reliable.
If you already have
a 690E you could make this bike suitable very simply.