Full frame lens make abbsolutly no difference to the reach you'll get. Yes the K7 will crop the edges of a full frame image giving a similar field of view to 1.5 times the focal length acheives on a full frame, but that applies to all lenses.
The Pentax DA55-300 will give you just as much reach, at the long end & goes a little wider. I havent used the Sigma70-300 but I'm fairly sure it will be significantly heavier.

I use the Bigmos on my K7 (weight can be a real issue) I'm not convinced the stabilized viewfinder makes that much difference, and switching the K7's SR off is a bit of a pain if you're switching between lenses a lot. Mind you my other 500mm+ lenses have all been manual focus so I've not been trying to hold a focus point on target, so the OS might be worth the extra hassle, but I've never missed it on any of my 300's

After reading your first line I expected to damage to the front of the bike

Like Frogfish I prefer the top one, but that may be just that the pano needs to be bigger to get the impact it deserves.

If luggage capacity is becoming a factor you could always send some of your clothes back by post to make room for important stuff

Fish from the Irish sea are more likely to be an issue for you - but as Kris says there's no need to worry. The levels are very low & observations of the recovery around Chernobyl have lead to some controversial suggestions that constant moderately high background radioactivity may even be beneficial...

A quick investigation suggests the activity within 1cm of the most active is ~2µSv/hr - about 1/5th the extra activity you'd be exposed to when flying at 30,000ft. Professional airline pilots flying at this altitude roughly 65hrs a month supposedly have a roughly 1% increased risk of cancer, so if you're very much in love with your Takumar lens you MIGHT be get a measurable increased risk - I wouldn't want to sleep with my lenses though, they might get fungus

It also seems that the initial radiation IS alpha (for both isotopes of Thorium used in the glass), this is all stopped by other lens elements. After undergoing radioactive decay the subsequent decay steps can involve beta radiation.

This Link on Thoriated camera lenses might be interest.

Typical low energy alpha particles would be of little concern in this sort of application as they are stopped by the thin layer of dead skin on the outside of your body, or a few cm of air, the glass of the viewfinder would be quite enough to stop any getting to your eye. They only become a significant hazard internally, as there they readily contact living cells. Active dust is probably the most serious hazard with alpha radiation as this opens up the possibly of ingestion/inhalation.

I don't have actual data on the activity of the glass used in those Takumar lenses, but I suspect the levels are exceeded my many everyday situations. Living in Cornwall, flying, eating brazil nuts... all significantly increase your background exposure to radioactivity.

Beta & gamma radiation travel further (a few mm of lead stops beta) but are less ionising.

I'd expect both types of sensor used in cameras (CMOS & CCD) to record radioactive particles in the same way that film will. Occasionally such interactions will damage the sensor giving either a hot pixel or a dead one. Quite a lot of random pixels can be effected like this before you're likely to notice unless you go looking for them.

If only my stopages were only 48p a week - I might be able to afford living in the manner to which I've become acustomed.
When I started here 25 years ago, I ended up with about £400 a month spending money, these days despite a four fold increase in basic pay, it's probably nearer to spending £400 a month more than I earn! (Mortgage, family... ...no longer young, free & single)

I brought some very cheap rechargable Li ion (CR7?) batteries for my K100. Before they arrived I read reports that there are two type of these the newer ones being OK, whilst the older give higher voltages(the e-bay add actually listed them as OK for the K100). When I got them I checked the voltage 4.7V each potentially harmful to circuitry designed for the 2 AA they replace (~3.6V for new alkalines).

Given they cost £20 (2 batteries with charger) it wasn't worth sending them back to china, they'll get used sometime in something less sensitive (old flash/toy car...)

Where hardware is designed for Li ion batteries I wouldn't expect this to be an issue, but it's probably still best to avoid the very cheapest options.

Reading the original post again, I see you say you want flexibility...
So some other options to consider beyond longer telephotos:

A fast 50 (or wider) prime for low light shots? Depending on type (Manual or auto aperture/focus) These can be had from ~£20 to ~£150+

A Macro for ultra close-ups From £50 for older models.

Ultra wide angle/Fisheye typically >£300.

Any lens with a PK mount can be used with the K10D (or any Pentax DSLR) lenses having M42 screw mounts, T2 mounts, or Adaptall2 mounts can be used with suitable adapters.

Earlier mounts (PK rather than PKA)along with nearly all adapted lenses will require manual apertures (green button for metering) and manual focusing.

PKA mounts (A series lenses from Pentax & third party ranges) give auto aperture (when the aperture ring is on A) but remain manual focus.

Auto focus lenses should all work fine, wether screw type or ultra sonic (USM/HSM). Pentax F & FA series were designed for film but fit this group. DA lenses are designed for digital (Due to lower coverage they often wont usually cover a 35mm negative)

If funds are tight you might want to consider older lenses.
I had a 500mm mirror lens (Manual focus & fixed aperture) for many years, but found the F100-300 (a bit heavy) generally gave better pictures, both are often seen for under £100.
The DA55-300 is a cheapish & reasonable quality modern lens to consider too...

Must go to work.